THE   ORE   DEPOSITS  OF  SOUTH    AFRICA 


THE  ORE   DEPOSITS 

OF 

SOUTH   AFRICA 

WITH   A  CHAPTER   ON 

HINTS  TO   PROSPECTORS. 

BY 

J.   P.  JOHNSON, 
// 

of  Council  of  Geological    Society  of  South  Africa, 
Certificated  Mine  Surveyor  of  the  Transvaal, 
Kiiif/'*  Prizeman  in  Metallurgy. 

PAHT   1. 

BASE    METALS. 

WITH   1)  Lid  HAMS. 


NEW   YOEK: 

THE   NOEMAN    W.   HENLEY   PUBLISHING   CO., 
132,   NASSAU    STREET. 

LONDON: 

CROSBY     LOCKWOOD     AND     SON, 
1908. 


Postal  &  Cable  Address,  Box  6231. 

J.    P.    JOHNSON, 

mining  Geologist 
=  and  engineer,  = 


Mines  Sampled,  Surveyed,  and    Valued. 

33,  Permanent  Buildings,  Harrison  Street, 
JOHANNESBURG. 


PREFACE. 


THIS  little  volume  is  intended  to  meet  a  demand  among 
those  technically  connected  with  the  mining  industry,  for  a 
co-ordinated  and  condensed  account  of  the  ore-deposits  at  pre- 
sent known  in  South  Africa. 

It  is  also  intended  as  a  guide  to  the  prospector.  Only  an 
elementary  knowledge  of  geology  and  some  mining  experience 
are  necessary  in  order  to  understand  it.  With  these  qualifica- 
tions, it  will  materially  assist  him  in  his  search  for  metallifer- 
ous mineral  occurrences  and,  so  far  as  simple  ores  are  con- 
cerned, should  enable  him  to  form  some  idea  of  the  possibili- 
ties of  any  he  may  find.  Regarding  complex  ores  he  will,  of 
course,  need  metallurgical  advice  in  addition. 

Throughout  this  book  the  stratigraphy  of  the  country  is 
ignored,  the  principles  of  ore  deposition  being  independent  of 
it.  Moreover,  it  is  already  dealt  with  in  Hatch  and  Corstor- 
phine's  "Geology  of  South  Africa." 

Unless  otherwise  stated  the  writer  has  seen  the  occurrences 
described. 

P.O.  Box  6231,  JOHANNESBURG, 
August,  1908. 


CONTENTS. 


PAGE 

CHAPTER  I.— INTRODUCTION      ...  1 

PRIMARY  ORES               ..          ...          ...          ...  1 

SECONDARY  ORES         ...          ...          ...          ...  8 

SULPHIDE  SEGREGATIONS          . .          ...          ...  4 

DYKE  SEGREGATIONS  AND  PEGMATITES          ...  5 

CLASSIFICATION  OF  ORE-OCCURRENCES            ...  6 

CLASSIFICATION  OF  IGNEOUS  ROCKS  ...          ...  7 

CHAPTER  II.  —  TlTANIFEROUS  AND  CHROMIFEROUS  IRON  OxiDES  9 

CHAPTER  III.— NICKEI 12 

CHAPTER  IV.— COPPER 18 

CHAPTER  V.— COBALT 28 

CHAPTER  VI.— TIN       30 

CHAPTER  VIL— MOLYBDENUM... 38 

CHAPTER  VIII.-  TUNGSTEN     40 

CHAPTER  IX.-  LEAD 41 

CHAPTER  X.- MERCURY            ...                      46 

CHAPTER  XI.— ANTIMONY         ...                      47 

CHAPTER  XII.-lRON 49 

CHAPTER  XIII— HINTS  TO  PROSPECTORS        ...                       ...  50 

ANTIMONY             ...          ...          ...          ...  51 

ARSENIC  ...          ...          ...          ...          ...  52 

CADMIUM...          ...          ...          ...          ...  52 

CHROMIUM            ...                       ...          ...  52 

COBALT  AND  NICKEL       53 

COPPER    ...          ...         ...         ...          ...  53 

LEAD        ...          ...          ...          ...          ...  54 

MANGANESE          54 

MERCURY...         ...          ...          ...         ...  55 

MOLYBDENUM       ...          ...          ...          ...  55 

TANTALUM            55 

THORIUM 55 

TIN          56 

TUNGSTEN            56 

URANIUM...         ...         ...          ...          ...  56 

VANADIUM            57 

ZINC         ...  57 


The   Ore-Deposits   of   South   Africa, 


CHAPTER  I. 

INTRODUCTION. 

Ax  understanding  of  the  genesis  of  an  ore  occurrence  is 
essential  before  any  exploitation  of  it  is  attempted,  since  it 
alone  enables  one  to  answer  the  all-important  questions, 
whether  it  will  continue  in  depth  and  whether  its  value  will 
increase  or  decrease.  Our  knowledge  of  the  processes  of  ore- 
formation  is  now  sufficiently  advanced  to  enable  us,  in  the 
majority  of  cases,  to  answer  these  questions  with  confidence, 
though  in  other  instances  it  is  still  impossible  to  do  so. 

PRIMARY  ORES. 

The  investigations  of  the  last  few  years  into  the  genesis  of 
ore-occurrences  have  resulted  in  the  almost  universal  accept- 
ance of  the  theory  that  they  have  a  magmatic  origin,  that  the 
deep-seated  magmas  contain  water  and  dissolved  metals  which , 
upon  the  ascent  of  the  magma  into  a  zone  of  lessening  pres- 
sure, escape,  penetrate  the  overlying  rocks,  and  even  reach 
the  surface  as  thermal  springs.  The  immediately  resulting 
ore-occurrences  are,  in  order  of  formation  :  (1)  Segregations 
within  the  magma ;  (2)  Impregnations  at  or  near  the  point 
where  the  volatile  substances  left  the  magma ;  and  (3) 
Deposits  by  waters  condensed  at  greater  distances  from  the 
magma  on  their  way  to  the  surface.  No  better  illustration  of 
this  theory  could  be  instanced  than  the  great  Bushveld  lac- 
colith, which  is  probably  unique  among  plutonic  masses  in  the 


2    /i\;   ^ij^Jt.Jwi  Dejjositv  of  South  Africa. 

extent  to  which  its  structure  has  been  revealed  by  denudation , 
and  in  the  slight  amount  of  distortion  to  which  it  has  been 
subjected  by  subsequent  earth  movements.  This  igneous  com- 
plex consists  of  a  dominant  granite  core  which  passes  along 
its  circumference,  through  syenites  and  gabbrodiorites,  into 
pyroxenite  containing  ore-bodies  of  the  first  class,  namely, 
segregations  of  titaniferous  and  chromiferous  iron  oxides.  The 
rock  masses  overlying  the  central  granite  portion,  as  well  as 
the  upper,  and  earlier  solidified,  part  of  the  granite  itself,  are 
impregnated  with  tin  and  copper  minerals,  constituting  ores 


FIG.  1. 
Section  at  Blaafjeld,  Scandinavia  (after  Voyt). 

Syenite  with  2  per  cent,  of  ilmenite,  containing  basic  pegmatite  with  40 
per  cent,  of  ilmenite  and  segregated  masses  of  ilmenite. 

of  the  second  class.  While  the  copper-lead-zinc  veins  in  the 
surrounding  strata  afford  excellent  examples  of  the  third  class. 
The  kinship  between  a  metalliferous  segregation  in  pyroxenite 
and  a  metalliferous  deposit  in  a  fissure  is,  no  doubt,  very  dis- 
tant, but  it  is  nevertheless  real. 

In  considering  the  sequence  of  these  ore-occurrences  it 
should  be  borne  in  mind  that  the  exterior  of  the  intruded 
magma  will  solidify  before  the  interior ;  that  the  molten  por- 
tion of  the  latter,  with  its  periodically  escaping  residual  water 
and  dissolved  metals,  would  recede  more  and  more  from  the 
surface  as  the  whole  gradually  cooled ;  and  that  what  was  in 


Introduction.  3 

:li<  earlier  stages  a  zone  of  gaseous  impregnation  might  at  a 
later  period  become  a  zone  of  deposition  from  solution. 
Further,  it  is  probable  that  at  different  levels  in  the  super- 
incumbent rocks  penetrated  by  the  ascending  vapours  and  solu- 
tions, different  chemical  reactions  take  place  according  to  the 
pressure  and  also  to  the  constitution  of  the  rock,  involving  the 
precipitation  of  different  groups  of  minerals. 

SECONDARY  ORES. 

At,  and  within  a  comparatively  short  distance  of,  the  sur- 
face, ore-bodies  are  subject  to  the  disintegrating  influences  of 
the  atmosphere  and  of  descending  surface  waters.  This  dis- 
integration, which  is  chiefly  due  to  the  presence  of  oxidizable 
minerals,  usually  extends  more  or  less  completely  down  to  the 
level  of  the  ground  wrater,  and  its  intensity  is  dependent  on 
the  amount  of  these  oxidizable  minerals  present.  In  most 
ore-bodies  the  original  valuable  minerals  are  sulphides  wrhich 
are  all  readily  oxidized,  and  many  of  these  are  converted  into 
soluble  compounds,  which  are  carried  down  by  the  descending 
waters  and  reprecipitated  at  the  bottom  of  the  porous  zone  of 
oxidation,  in  the  form  of  carbonates,  chlorides,  oxides  and 
silicates.  In  some  cases  the  compounds  in  solution,  on  reach- 
ing the  unoxidized  zone,  are  reconverted  into  sulphides. 

Thus  three  zones  are  frequently  encountered  in  opening  up 
such  ore-bodies,  namely,  (A)  A  barren  zone  of  disintegrated 
rock,  termed  the  gossan,  (B)  A  high-grade  layer  of  oxidized  or 
sulphide  ore,  knowrn  as  the  zone  of  secondary  enrichment,  and, 
(C)  The  main  body  of  primary  low-grade  sulphide  ore.  Fig.  2 
shows  an  actual  example. 

In  other  ore-bodies  where  the  valuable  mineral — cassiterite 
for  instance — is  insoluble,  no  such  chemical  concentration 
takes  place,  but  rich  detrital  accumulations  may  result  from 
their  denudation. 

Most  of  the  metalliferous  occurrences  being  mined  at  the 
present  day  are  secondary  ore-bodies  of  the  above  kinds. 


4  Ore  Deposits  of  South  Africa. 

SULPHIDE  SEGREGATIONS. 

In  view  of  the  fact  of  magmatic  differentiation  there  is  noth- 
ing astonishing  in  finding  segregations  of  the  oxides,  ilmenite, 
magnetite  and  chromite,  since  all  are  common  constituents  of 
basic  igneous  rocks.  Now  in  Scandinavia,  Canada  and  South 
Africa,  sulphide  aggregates  (pyrrhotite,  pyrite  and  chalcopy- 
rite,  containing  a  small  percentage  of  nickel  and  cobalt)  have 
been  discovered  with  such  strikingly  similar  associations,  that 


FIG.  2. 

Section  through    Utah  and   Boston  Copper  Mines,  Binyham,  U.S.  (after 

Ingalh). 

(c)  Quartz -porphyry  dyke  impregnated  with  primary  sulphide  (chalcopy- 
rite).  Copper  contents  about  1  per  cent,  (a)  Leached  zone  stained  brown 
with  iron  oxide  and  mottled  with  green  and  blue  copper  carbonates.  Unpay- 
able. Thickness  30  to  150  feet,  (b)  Zone  of  secondary  sulphide  (chalcocite) 
enrichment.  Copper  contents  2  per  cent.  Thickness  100  to  300  feet. 


they  are  regarded  by  many,  with  good  reason,  as  magmatic 
segregations  also.  Others,  however,  maintain  that  they  are 
merely  ordinary  impregnations.  There  seems  to  be  one  point 
of  difference,  namely,  that  while  the  oxide  segregations,  though 
usually  near  the  margin,  are  always  distinctly  within  the 


Introduction.  5 

igneous  rock,  the  sulphide  aggregates,  on  the  other  hand,  are 
right  at  the  contact  of  country  and  intrusive  rock,  while  this 
last  is  much  altered  there.  The  feature  certainly  lends  some 
support  to  the  argument  of  the  objectors.  In  both  cases  the 
igneous  rock  is  similar  and  a  gradual  passage  can  be  seen 
between  it  and  the  metalliferous  masses.  The  absence  of 
undoubted  original  sulphides  in  igneous  rocks  presents  another 
obstacle  to  this  interpretation  of  the  occurrence.  Oxidizing 
conditions  were  obviously  the  rule  in  fused  magmas.  At  the 
same  time  there  may  have  been  exceptions.  It  has  been 
suggested,  for  instance,  that  the  escape  of  sulphurous  gas 
through  the  still  molten  rock  along  the  contact  converted  the 
contained  metals  into  sulphides.  The  remarkable  identity  of 
the  occurrences  in  such  widely  separated  parts  of  the  world, 
taken  in  conjunction  with  their  undeniable  resemblance  to 
undoubted  oxide  segregations,  makes  it  highly  probable  that 
they  are,  indeed,  segregations  also. 

DIKE  SEGREGATIONS  AND  PEGMATITES. 

Dykes  are  offshoots  from  deep-seated  magmas  that  have 
welled  -up  fault  planes  during  periods  of  disturbance.  They 
are  similar  in  composition  to  the  portion  of  the  magma  from 
which  they  come.  Magmatic  segregations  of  metalliferous 
minerals  are  hence  sometimes  met  with  in  the  form  of  dykes. 

Those  very  coarse-grained  aggregates  of  quartz  and  felspar, 
or  of  felspar  and  pyroxene,  with  or  without  accessory  minerals, 
known  as  pegmatites,  which  are  so  frequently  met  with  in 
granite  and  gabbrodiorite  areas,  and  which  are  chiefly  noted 
as  the  source  of  commercial  mica,  have  long  been  the  subject  of 
speculation.  Though  generally  regarded  as  modifications  of 
the  main  body,  it  is  maintained  by  many  that  pegmatites, 
whether  in  the  form  of  irregular  masses  or  of  vein-like  dykes, 
are  not  differentiations  of  the  magma,  but  have  crystallized 
under  high  pressure  from  the  superheated  aqueous  solutions 
that  remained  after  its  solidification.  In  other  words,  that 


6  Ore  Deposits  of  South  Africa. 

they  occupy  a  position  intermediate  between  igneous  masses 
and  mineral  veins. 

If  the  conception  of  the  genesis  of  ore  occurrences  pre- 
viously described  be  correct,  it  is  clear  that  many  substances 
are  more  soluble  in  such  residual  waters  than  in  the  magma 
itself.  Examples  of  these  are  tourmaline,  fluorite,  cassiterite 
and  allied  minerals.  Now  many  acid  pegmatites  actually  con- 
tain these  minerals  while  their  very  coarse  crystallization  is 
such  as  would  result  from  the  slow  cooling  necessary  if  the 
above  contention  be  correct. 

In  view  of  the  similarity  in  constitution  between  granites 
and  certain  mineral  veins,  for  instance,  those  of  the  Eooi  Berg, 
this  is  certainly  an  attractive  theory.  The  possibility  of  the 
tourmaline,  fluorite,  cassiterite,  etc.,  in  the  pegmatites,  being 
of  a  secondary  nature,  however,  must  not  be  overlooked.  In 
fact,  a  great  deal  of  wTork  will  have  to  be  done  before  this 
explanation  of  the  origin  of  pegmatites  can  be  proved  or  dis- 
proved. 

CLASSIFICATION  OF  ORE-OCCURRENCES. 

A  number  of  classifications  of  ore-occurrences  have  been 
proposed  in  recent  years,  but  none  of  them  can  be  said  to  be 
satisfactory  from  the  point  of  view  of  the  mining  man.  In  the 
following  classification  the  writer  has  attempted  to  remedy 
this  defect.  The  object  of  classification  in  scientific  and  tech- 
nical subjects  is  to  enable  the  student  to  more  readily  obtain  a 
firm  grasp  of  his  subject  and  of  the  interrelation  of  its  several 
branches.  In  the  imparting  of  knowledge  it  may  be  compared 
to  organization  (as  opposed  to  muddle)  in  other  branches  of 
human  endeavour.  Such  a  classification  must  present  a  broad 
view  unobscured  by  minor  details,  and  is  necessarily,  to  a 
certain  extent,  arbitrary.  Actually  no  hard  and  fast  line  can 
be  drawn  between  the  different  types  of  ore-occurrences.  In 
such  a  classification,  too,  the  main  sub-divisions  at  least  should 
be  of  a  kind  that  would  be  recognizable  in  the  field  ^vithout 


Introduction.  7 

recourse  to  laboratory  investigations.  The  following  is  the 
proposed  scheme  in  brief ;  the  relation  of  the  sub-divisions  to 
the  terms  Eeef  and  Lode  being  indicated  also.  By  ore  is 
meant  any  metalliferous  mineral  or  rock  of  present  or  prospec- 
tive economic  value. 

A.  Magmatic  Segregations. 

B.  Irregular  Impregnations. 

C.  Regular  Impregnations  of  a  Stratum. 

D.  „  „  of  a  Dyke. 

E.  ,,  ,,  along  contact  with 


Reefs. 


a  Dyke. 


VLodes. 


F.  ,,  ,,  of   the    walls    of    a 

fissure. 

G.  Regular  Gratifications*  (in  fissures) 
H.  Irregular  Gratifications  (in  pockets). 
I.     Detrital  Accumulations. 

The  great  majority  of  ore-occurrences  fall  under  the  head- 
ing of  Impregnations  and  Crustifications.  Impregnations  are 
those  in  which  the  metalliferous  and  accompanying  minerals 
are  disseminated  throughout  a  pre-existing  rock.  Crustifica- 
tions are  those  in  which  the  metalliferous  and  accompanying 
("  gangue  ")  minerals  occupy  cavities  in  a  rock. 

CLASSIFICATION  OF  IGNEOUS  ROCKS. 

The  classification  of  the  igneous  rocks  here  employed  is 
that  recommended  for  field  use  by  the  authors  of  the  Quantita- 
tive Classification  of  the  Igneous  Rocks.  It  is  a  classification 
based  entirely  on  megascopic  characters  which,  however,  in  a 
broad  way  reflect  their  chemico-mineralogical  constitution. 
The  mining  geologist  seldom  requires  to  define  an  igneous 
rock  more  exactly,  and  when  it  is  necessary  to  do  so,  a  descrip- 
tion of  its  microscopical  characters  will  serve  the  purpose. 

*  Crustifications  —  "  Fillings  of  Fissures  and  other  Cavities,"  Posepny, 
Genets  of  Ore  Deposits,  p.  12  ^1905;. 


8  Ore  Deposits  of  South  Africa. 

The  primary  sub-division  is  into  phanerites  and  aphanites. 
Phanerites  are  those  whose  constituent  minerals  are  sufficiently 
large  to  be  visible  to  the  unaided  eye.  Aphanites  are  those 
whose  constituent  minerals  are  too  small  to  be  visible  to  the 
naked  eye.  Those  rocks,  intermediate  between  phanerites 
and  aphanites,  which  consist  of  megascopic  minerals  dis- 
tributed throughout  a  groundmass  of  microscopic  constituents 
are  termed  porphyries. 

The  major  bulk  of  the  phanerites  may  be  referred  to  three 
groups,'  (1)  Granite,  those  composed  essentially  of  quartz  and 
felspar,  (2)  Syenite,  those  composed  essentially  of  felspar  and 
amphiboles  or  pyroxenes,  the  former  being  dominant,  and 
(3)  Gabbrodiorite ,  those  composed  essentially  of  pyroxene  or 
amphibole  and  felspar,  the  latter  being  subordinate. 

In  addition  to  these  are  the  rare  felspar-less  phanerites 
which  are  named  according  to  the  dominant  constituent 
mineral,  e.g.,  Amphibolite,  Pyroxenite,  Peridotite,  etc. 

Since  quartz  and  the  felspars  are  light  in  colour  and  the 
pyroxenes  and  amphiboles  are  dark,  it  is  usually  possible  to 
infer  the  end  of  the  scale  to  which  an  aphanite  inclines. 
Hence,  this  factor — whether  acid  or  basic — should  be  men- 
tioned. 

The  porphyries  are  further  qualified  as  Quartz-porphyries, 
Felspar-porphyries,  Pyroxene-porphyries,  etc.,  according  to 
the  mineral  megascopically  developed. 


CHAPTER  II. 

TITANIFEROUS   AND   CHROMIFEROUS    IRON    OXIDES. 

BUSH  VELD  LACCOLITH  (A*). 

SEGREGATIONS  of  titaniferous  and  chromiferous  iron  oxides 
occur  in  the  basic  margin  of  the  Bushveld  laccolith,  and 
although  not  at  present  of  commercial  value  as  iron  ores,  since 
both  the  titanium  and  chromium,  in  the  quantities  in  whicli 
they  occur  in  these  ore-bodies,  cause  serious  difficulties  in  their 
treatment, t  must  yet  sooner  or  later  come  within  the  range  of 
payability,  either  through  scientific  investigation  overcoming 
the  difficulties  or  through  the  exhaustion  of  the  more  easily 
treated  ores. 

Enormous  bodies  of  the  titaniferous  ore  have  been  located 
all  along  the  periphery  of  the  laccolith.  The  mineral  is  mainly 
magnetite,  the  titanium  being  present  either  as  a  chemical 
combination  in  place  of  some  of  the  iron,  or  else  as  a  mechani- 
cal mixture  in  the  form  of  ilmenite  or  rutile. 

The  chromite  bodies,  though  probably  possessing  an  equally 
long  distribution  are  not  so  large.  They  have  been  well 
studied  along  the  southern  and  eastern  margins  of  the  lac- 
colith by  Hall  and  Humphrey,  while,  more  recently,  Schoch 
has  a'lso  added  to  our  knowledge  of  them.  They  are  associated 
with  very  basic  members  of  the  marginal  rocks,  among  which 
types  rich  in  pyroxenes  and  poor  in  felspars  are  common.  The 
ore  occurs  in  fairly  well  denned  sheets  with  a  dip  and  strike 

*  The  letters  in  brackets  indicate  the  position  of  the  ore-occurrence  in 
the  scheme  of  classification.  In  this  case  for  example  (A)  =  Magmatic 
Segregation. 

t  Although  both  titanium  and  chromium  impart  desirable  properties  to 
steel  when  added  in  certain  small  proportions. 


10  Ore  Deposits  of  South  Africa. 

roughly  parallel  to  that  of  the  periphery  of  the  laccolith.  In 
thickness  they  range  from  five  feet  downwards,  and  usually 
maintain  a  fairly  uniform  width  for  some  distance. 

In  its  purest  form  the  ore  consists  of  a  black  lustrous  fine- 
grained aggregate  of  chromite,  which  easily  weathers  to  a 
friable  rock  and  becomes  readily  disintegrated  into  a  heavy 
dark-coloured  sand.  As  the  ore  becomes  less  pure  pyroxenes 
appear,  and  the  gradual  transition  can  be  traced  from  black, 
heavy  chromite-ore  to  less  heavy  and  dark-coloured  pyroxenite, 
containing  scattered  grains  of  chromite,  and  thence  into  gab- 
brodiorite. 

A  little  platinum  is  associated  with  the  chromite. 

The  southern  belt  stretches  from  north-west  of  the  village 
of  Eustenburg  to  the  Crocodile  River,  a  distance  of  about 
twenty-eight  miles.  The  farms  on  which  the  chromite  has 
been  located  are  :  Boschkopje,  Boschfontein,  the  Rustenburg 
Commonage,  Waterval,  Arnoldstad,  Kronendal,  Rietfontein, 
Kaffirskraal,  Dekroon,  Brakspruit,  Spruitf ontein ,  Rooikopjes, 
and  others.  The  eastern  belt  stretches  north-westwards  from 
the  Steelpoort  River,  and  chromite  has  been  located  on  the  fol- 
lowing farms  :  Hendricksplaats,  Mooihoek,  Mondagshol,  Twy- 
felaar,  Zwartkopjes,  Klipfontein,  Mooijalijk,  Brakfontein, 
Winterveld,  and  Jactslust. 

When  the  proportion  of  chromium  is  high  and  that  of  iron 
low,  this  ore  is  much  valued  for  its  chromic  oxide.  To  find  a 
market  it  must  contain  not  less  than  fifty  per  cent.  Ore  of 
that  standard  fetches  about  65s.  a  ton,  while  better  qualities 
command  a  premium.  The  laccolith  occurrences  hitherto  dis- 
covered do  not  come  up  to  the  market  requirements. 

SELUKWE  (A). 

Mennell  states  that  chromite  bodies  are  found  in  several 
places  along  the  great  peridotite  intrusion  that  runs  from  near 
Fort  Impatene  to  the  Zambesi,  notably  in  the  Hartley  and 
Lomagundi  districts.  Mining  is  carried  on  in  a  hill  of 


Titaniferons  and  Cliromiferous  Iron  Oxides.  11 

chromite  at  Selukwe.  This  occurrence  is  about  a  mile  from 
the  peridotite,  but  may  be  in  an  offshoot  from  it.  The  writer 
has  not  seen  this  ore-body.  The  following  outputs  have  been 
registered  :— In  1905,  276  tons;  in  1906,  3,371  tons,  and  in 
1907,  8,017  tons. 


CHAPTER  III. 

NICKEL. 
1NSIZWA  (A). 

Tms  nickel  occurrence  is  of  special  interest  in  that  it  be- 
longs to  the  class  of  magmatic  sulphide  segregations. 

The  Insizwa  range  is  situated  near  Mount  Ay  lilt.  It  is 
composed  of  a  big  gabbrodiorite  laccolith  intruded  into  a  series 
of  sandstones  and  shales.  The  igneous  rock  has  been  exposed 
by  denudation  all  over  the  upper  portion  of  the  range,  the 
sedimentary  formation  on  which  it  rests  constituting  the  lower 
slopes.  The  shales  are  much  altered  and  indurated  at  the  con- 
tact. 

The  mineral  is  pyrrhotite,  with  which  is  mixed  a  little 
chalcopyrite,  and,  in  very  subordinate  quantity,  pyrite.  The 
nickel  content  averages  about  seven  per  cent.  Traces  of  cobalt 
are  also  occasionally  present. 

The  sulphides  are  disseminated  throughout  the  marginal 
portion  of  the  gabbrodiorite,  and  increase  in  quantity  towards 
the  contact  where  they  form  lenticular  aggregates,  and  also 
slightly  impregnate  the  wall  rock. 

This  mode  of  occurrence  of  the  ore  is  strictly  homologous 
with  that  of  the  well  known  Scandinavian  and  Canadian  bodies, 
while  the.  comparative  freshness  of  the  igneous  rock  in  this 
instance  seems  for  the  first  time  to  place  sulphide  segregation 
beyond  doubt. 

The  occurrence  is  at  present  only  in  the  prospecting  stage, 
but  there  can  be  no  doubt  that  sooner  or  later  it  will  become  of 
economic  importance. 


CHAPTEE  IV. 

COPPER. 
OOKIEP  (A). 

THE  Ookiep  and  neighbouring  mines  are  situated  in  Little 
Namaqualand. 

The  writer  has  not  seen  them,  the  following  account  being 
derived  from  a  paper*  by  Kuntz  :— 

The  surrounding  district  is  occupied  by  granite  and  associ- 
ated schists.  This  formation  is  traversed  by  five  parallel  west 
and  east  lines  of  crush  along  which  dykes  of  gabbrodiorite  have 
risen.  It  is  in  these  dykes  that  the  copper  occurs. 

The  dykes  outcrop  prominently.  In  some  cases  they  are 
traceable  for  miles  without  break,  but  in  others  are  in  evidence 
only  from  point  to  point,  with  big  gaps  between. 

The  copper-bearing  minerals,  chiefly  bornite  and  chalcopy- 
rite,  are  disseminated  in  minute  quantities  throughout  the 
whole  of  the  gabbrodiorite,  and  in  spots  almost  entirely  replace 
it. 

While  the  entire  mass  of  the  gabbrodiorite  thus  contains 
traces  of  copper  and  rich  patches  occur  here  and  there  through- 
out, it  is  only  in  a  few  places  that  it  is  of  a  payable  nature. 
In  these  places  the  ore  occurs  in  the  form  of  very  large  and 
high-grade  pockets. 

Kuntz  considers  that  these  pockets  have  been  formed  at 
points  where  the  dykes  are  intersected  by  later  faults. 

On  the  most  southern  line  are  situated  the  abandoned  mines 
of  Spektakel  and  Springbok,  from  which  large  quantities  of 

*  J.  Kuntz,  Transactions  Geological  Society  of  South  Africa,  VII.,  p.  70- 
72  (1904). 


14  Ore  Deposits  of  South  Africa. 

high-grade  ore  were  obtained,  together  with  the  less  produc- 
tive workings  of  Koperberg. 

On  the  second  line  occur  the  mines  of  Nababeep,  Ookiep, 
and  Narap.  A  plan  and  section,  reproduced  herewith  (fig.  3), 
of  the  Ookiep  pocket  is  given  by  Kuntz  in  his  p&per.  Its  shape 
is  oblong,  with  a  peculiar  prolongation  along  the  cross  break. 

1 


FIG.  3. 

Plan  and  section  of  Ookiep  Copper  Nine  (after  Kuntz). 

Its  extent  from  west  to  east  is  nearly  a  thousand  feet,  its 
greatest  width  is  about  two  hundred  feet,  and  its  greatest 
thickness  (vertical)  three  hundred  feet. 

The  Nababeep  is  a  similar  enormous  ore-body,  while  Narap 
is  only  a  small  pocket  and  has  been  abandoned. 


Copper.  15 

On  the  fourth  line  lies  the  Tweefontein  mine,  which  is 
probably  the  most  instructive  to  the  geologist.  Kuntz  also 
gives  a  plan  and  section  of  this  mine,  while  Ronaldson*  has 
figured  a  further  section,  reproduced  herewith  (fig.  4),  made 
at  a  later  date.  It  consists  of  three  pockets.  Ore-bodies  No. 


TWEEFONTEIM 

MINE. 

Cross  Section-  of 
Ort-body  JK1 


FIG.  4. 
Section  through  Tweefontein  Copper  Mine  (after  Ronaldson). 

1  and  No.  2  are  situated  about  700  feet  apart  on  the  strike, 
both  outcropping  at  the  surface  but  at  different  elevations. 
Ore-body  No.  3  immediately  underlies  No.  2.  The  dimen- 
sions of  these  pockets,  in  the  order  given,  are  : — Greatest 

*  J.  H.  Ronaldson,  Trans.  Geol.  Soc.  S.  Africa,  VIII.,  p.  165  (1905). 


16  Ore  Deposits  of  South  Africa. 

length,  540,  550,  and  760  feet;  greatest  depth,  240,  225,  800; 
and  greatest  width,  75,  50,  and  60  feet. 

On  the  remaining  two  lines  are  a  number  of  abandoned 
mines  and  prospects. 

At  the  surface  the  copper  sulphides  are,  of  course,  mainly 
changed  to  malachite  and  azurite,  while  the  red  and  black 
oxides,  cuprite  and  tenorite,  and  the  green  silicate,  chrysocolla, 
as  well  as  copper  itself,  occur  in  small  quantity. 

Kuntz  considers  the  copper  sulphide  to  be  an  original  con- 
stituent of  the  dykes,  in  other  words,  that  the  dykes  are 
intruded  portions  of  a  magmatic  sulphide  segregation,  the  pay- 
able pockets  being  secondary  enrichments. 

The  following  figures  have  been  compiled*  of  the  output  of 
this  field  :  — 

From  the  opening  of  the  mines  in  1852  to  the  end  of  1862  : 
Springbok,  Spektakel,  and  Ookiep  produced  18,999  long  tons. 
From  1862  to,  1882  (20  years)  Ookiep  produced  164,025, 
Spektakel  14,765,  and  Trial  Mines  1,057.  The  average  cop- 
per content  of  this  ore  was  29 '5  per  cent. 

From  the  beginning  of  1883  to  the  end  of  1904  the  produc- 
tion in  long  tons  was  :  — 

Ookiep      534,626,  averaging  20'20  per  cent,  copper 

Spektakel       ,      ...       19,636  ,,         31/85 

Springbok  ...  332  ,,         30'50 

Nababeep  ...     114,332  ,,  6'16 

Ookiep,  E.          ...       19,022  ,,  5'02 

Xoperberg  ...         6,087  ,,          9'49 

Narap       1,326  ,,         13'05 

Twfeefontein        ...     138,683  ,,         25*40 

Flat    Mine          ...         5,000  ,,         20*00 

Hester  Maria      ...         3,500  ,,         20'00 

As  many  of  the  above  percentages  of  copper  contents  are 
those  of  the  ore  subsequent  to  concentration,  it  is  impossible  to 

*  T.  Quentrall  and  J.  H.  Konaldson,  Report  to  Government,  Capetown 
(1905). 


Copper.  17 

arrive,  approximately  even,  at  the  average  value  of  the  ore 
mined  and  raised,  but  much  of  it  from  Springbok,  Spektakel, 
and  Ookiep  mines  has  been  abnormally  rich. 

MESSINA  (F). 

The  Messina  copper  belt  is  situated  in  the  west  angle 
between  the  Zand  River  and  the  south  bank  of  the  Limpopo. 
It  is  nearly  twenty  miles  in  length  and  extends  in  a  north- 
easterly direction  from  the  "farm"  Oostenryk,  through 
Papenbril,  Vogelsang,  Berkenrode  (on  which  the  Messina  mine 
is  located)  and  Uitenpas,  to  Artonvilla.  It  consists  of  a  line 
of  crush  in  the  schistose  granite  country,  impregnated  with 
copper  minerals  and  contains  a  number  of  well-defined  fissures, 
the  walls  of  which  have  been  impregnated  and  here  and  there 
completely  replaced  by  lenticular  bodies  of  copper  sulphide. 
The  whole  belt  is  riddled  with  old  Kaffir  workings  wrhich  reach 
a  depth  of  seventy  feet  and  are  situated  on  lenses  of  rich  ore. 
They  are  probably  the  most  extensive  of  their  kind  in  South 
Africa,  and  many  thousand  tons  of  copper  must  have  been 
extracted  from  them. 

The  Messina  is  the  only  mine  so  far  opened  up  on  this  belt. 
Most  of  the  development  has  been  done  on  the  two  main  lodes 
whose  strike  is  indicated  by  the  two  chief  lines  of  old  workings 
shown  in  the  accompanying  plan  (fig.  5).  The  northern  of 
these  lodes  has  been  opened  up  for  a  length  of  800  feet,  and 
the  southern  for  500  feet,  the  maximum  depth  being  200  and 
300  feet  respectively.  The  accompanying  longitudinal  sec- 
tions (fig.  6)  through  them,  showing  the  bodies  of  rich  ore 
(20  per  cent,  copper)  so  far  encountered  and  stoped  out,  and 
those  extracted  at  the  surface  by  the  old  workers,  well  illustrate 
the  mode  of  occurrence.  The  mineral  encountered  so  far 
is  almost  entirely  chalcocite,  there  being  very  little  bornite 
or  chalcopyrite  and  hardly  any  of  the  oxidation  products 

(malachite,  etc.)  which,  however,  probably  constituted  the  ore 

c 


18  Ore  Deposits  of  South  Africa. 


o 

o 


I 


FIG.  5. 

o/  Old  Workings  at  Messina  Copper  Mine  (after  Callerwood}. 
A  few  are  not  shown,  being  hidden  by  modern  dumps. 


Copper. 


19 


^ 


i 


I 


20  Ore  Deposits  of  South  Africa. 

extracted  from  the  old  workings.  The  richer  ore  contains  a 
few  ounces  of  silver  to  the  ton  and  traces  of  gold. 

The  development  has  clearly  not  yet  reached  the  bottom 
of  the  zone  of  secondary  sulphide  enrichment,  which  in  this 
case  extends  practically  to  the  surface. 

The  following  amounts  of  picked  ore  containing  61  per- 
cent, copper  have  been  shinned: — In  1906,  585  tons;  1907, 
1,242  tons;  1908,  January-June  inclusive,  370  tons. 

SWAKOP-KUISEB   (E). 

A  number  of  copper  occurrences  have  been  located  in  the 
schist  country  between  the  Swakop  and  Kuiseb  Rivers.  The 
writer  has  not  visited  them,  but  they  have  been  well  described 
in  a  paper*  by  Voit,  from  which  the  following  notes  have  been 
extracted  :  — 

The  most  extensive  occurrences  are  those  at  the  Hope, 
Gorap,  and  Matchless  mines.  All  these  are  situated  in  close 
proximity  to  a  belt  of  schistose  amphibolite  which  strikes 
through  the  district  in  a  north-easterly  direction  and  which 
seems  to  be  connected  with  the  origin  of  the  ore-bodies. 
Voit's  paper  is  illustrated  by  plans  and  sections  of  these. 

At  the  Gorap  mine  the  copper  occurrences  are  situated 
within  a  huge  lenticule  of  mica -schist  enclosed  by  the  amphi- 
bolite. In  this  mica-schist  is  an  interbedded  string  of 
quartzite  lenticules  with  a  length  of  strike  of  about  three  miles. 
It  is  in  the  hanging  wall  of  these  quartzite  lenticules  that  the 
copper  impregnations  occur. 

The  ore  is  chalcopyrite  and  chalcocite,  which  give  way  at 
the  surface  to  a  reddish-brown  mixture  of  cuprite  and  iron- 
stone mottled  here  and  there  with  the  green  and  blue  copper 
carbonates.  Volborthite  also  occurs  in  yellow  to  green  earthy 
masses. 

At  the  Hope  mine  the  copper  occurrence  is  very  similar. 
There  are  two  parallel  quartzite  lenticules  interbedded  with 

*  F.  W.  Voit,  Trans.  Geol.  Soc.  S.  Africa,  VII.,  p.  77-94  (1904). 


Copper.  -2\ 

• 
the   Dlica-sciiial.     .between    these   arc   three   zones,   having   a 

length  of  about  '2(50  feet  and  a  width  of  about  one  foot,  impreg- 
nated with  copper-ore. 

At  the  Matchless  mine,  which  is  about  130  miles  away 
from  the  Hope,  two  bands  of  amphibohte  enclose  a  belt  of 
mica-schist  and  schistose  granite.  In  these  schists  is  a  large 
lenticule  of  quartzite  on  the  hanging  wall  of  which  the  im- 
pregnations of  copper  ore  occur.  The  mineral  is  chiefly  chal- 
copyrite,  which  is  changed  at  the  surface  to  green  copper  car- 
bonate and  brown  iron-oxide,  but  solid  layers  of  bornite  and 
chalcocite  are  also  present. 

Voit  remarks  that  ' '  the  ores  occur  in  compact  masses 
chiefly  at  the  surface  and  for  a  short  depth  down  only,  whilst 
at  a  greater  depth,  where  the  bedding  was  able  to  offer  a 
greater  resistance  to  the  circulation  of  waters,  these  ores 
gradually  thin  out  to  narrow  seams,  and  finally  pass  into  finely 
disseminated  impregnations."  From  which  it  is  clear  that  the 
payable1  ore  is  a  secondary  concentration  of  a  low-grade  im- 
pregnation. 

EDMUND  I  AN  (F). 

The  Edmundian  mine  is  situated  about  190  miles  by  rail 
from  Beira,  about  seven  miles  west  of  Macequece,  and  about 
ten  miles  east  of  Umtali.  It  lies  near  the  top  of  a  low  range  of 
mountains  rising  up  from  the  north  side  of  the  Beira-Mashona- 
land  railway,  with  which  it  is  connected  by  a  good  wragon 
road  of  about  two  miles  in  length,  with  a  steady  downward 
uiade.  The  mode  of  occurrence  of  the  ore  has  been  well 
described*  by  Brackenbury.  It  consists  of  lenticular  bodies 
of  chalcopyrite,  with  a  little  pyrrhotite,  and,  in  the  upper  por- 
tion of  the  mine,  their  oxidation  products,  distributed  in  the 
form  of  vertical  shoots  throughout  a  zone  of  fracture  in  amphi- 

*  C.  Brackenbury,  Trans.  Institution  of  Mining  and  Metallurgy,  XV.' 
p.  633-642  (1906). 


22  Ore  Deposits  of  South  Africa. 

bolic  and  chloritic  schists.  The  rich  shoots  often  attain  a 
thickness  of  over  three  feet  of  solid  sulphide,  narrowing  down 
laterally  to  veins  a  few  inches  wide,  which  in  turn  split  up 
into  little  stringers  and  finally  into  disseminations  along  the 
course  of  the  lode. 

UMKONDO  (C). 

This  is  a  promising  proposition  situated  some  115  miles 
east  of  Victoria  by  wagon  road,  about  ten  miles  west  of  the 
Sabi  River,  and  some  130  miles  south  and  slightly  west  of 
Umtali. 

Its  site  was  indicated  by  the  presence  of  numerous  old 
workings,  which  extend  over  a  length  of  about  1,500  feet,  and 
are  in  places  over  300  feet  wide.  The  old  workers  not  only 
made  large  open-cast  excavations,  as  much  as  60  feet  in  depth, 
but  also  sank  small  shafts,  and  in  places  sloped  out  the  ore 
right  down  to  the  water  level. 

The  copper  occurs  in  a  sedimentary  formation,  consisting 
of  much  altered  shales  and  sandstones  overlying  granite, 
apparently  unconformably,  and  intersected  by  numerous  gab- 
brodiorite  dykes. 

Brackenbury,  who  gives  a  very  able  and  detailed  descrip- 
tion of  this  occurrence,  concludes  that  the  copper  has  been 
subsequently  introduced  into  these  beds  along  zones  of  frac- 
ture, and  that  although  the  ore  in  some  cases  occurs  as  a 
bedded  deposit,  it  is  not  likely  to  be  found  in  this  form  at  any 
great  distances  from  these  zones  of  disturbance.  He  also  con- 
nects the  ore-bodies  with  the  gabbrodiorite  intrusions. 

The  present  water  level  has  been  found  to  be  less  than  one 
hundred  feet  belowr  the  surface,  but  the  sulphides  do  not 
appear  until  some  distance  below  this. 

In  the  zone  of  oxidation  the  ore  is  mainly  green  and  blue 
carbonate  in  the  form  of  incrustations  and  little  seams  in  the 
bedding  planes  of  the  shales  and  in  the  fractures  in  the 


Copper.  23 

quavtzite,  bat  also  occurs  as  an  impure  grey  oxide  in  the  shape 
of  hard  nodules  in  the  shale  up  to  several  inches  in  diameter. 

ALASKA  (B). 

This  is  a  promising  prospect ,  situated  some  90  miles  north- 
west of  Salisbury,  about  thirty  miles  west  of  the  Ayrshire  mine 
and  only  some  thirteen  miles  from  the  Eldorado  mine. 

It  is  remarkable  for  the  great  size  of  its  old  workings, 
which  are  perhaps  the  largest  in  the  country  between  the  Lim- 
popo and  Zambesi  Elvers.  They  extend  in  an  almost  un- 
broken line  for  about  1,700  feet,  with  an  extreme  width  of  660 
feet,  and  they  have  been  proved  to  go  down  in  some  places  to  a 
depth  of  at  least  70  feet. 

The  occurrence  has  been  well  described  by  Brackenbury 
in  the  paper  already  cited.  The  copper  so  far  exposed  is 
mainly  in  the  form  of  the  green  carbonate,  malachite,  the  sul- 
phide zone  not  yet  having  been  reached.  It  is  disseminated 
throughout  siliceous  and  calcareous  schists. 

PALABORA  (B). 

The  Palabora  kopjes  are  situated  some  forty  miles  east  of 
Leydsdorp  and  are  celebrated  as  the  source  from  which  the 
Kafirs  of  the  surrounding  districts  formerly  obtained  their  iron 
and  copper  ores.  I  visited  the  locality  in  1907,  and  since  then 
a  detailed  account  of  the  old  workings  has  been  published  by 
Mellor.*  The  kopjes,  with  one  exception,  to  which  the  name 
Lulu  has  been  given,  are  of  granite,  and  stand  out  conspicu- 
ously from  the  surrounding  flat  or  slightly  undulating  bush- 
country.  Lulu  kop  is  composed  of  a  white  metamorphosed 
limestone  or  marble,  and  it  is  in  this  that  the  ore-bodies  and 
old  workings  are  mostly  situated. 

Magnetite  is  distributed  almost  everywhere  throughout  the 

*  E.  T.  Mellor,  Report  Geological  Survey,  Pretoria,  p.  44-49  (1907). 


24  Ore  Deposits  of  South  Africa. 

limestone,  occurring  in  grains,  crystals  and  masses,  up  to  six 
feet  or  more  in  diameter.  ]t  is  especially  abundant  along  cur- 
tain bands  in  the  limestone. 

Chalcopyrite  is  found  in  places  disseminated  in  small 
quantity  throughout  the  limestone  while  its  alteration  pro- 
ducts, malachite  and  azurite,  are  more  abundant,  though  only 
constituting  a  very  small  proportion  of  the  rock. 

The  old  workings  extend  over  the  whole  area  of  the  kop 
and  are  mostly  shallow  pits  which  have  been  sunk  on  pockets 
of  ore. 

Nothing  of  a  payable  nature  is  exposed,  but  indications  of 
copper  are  so  abundant,  not  only  in  the  limestone,  but  in  the 
adjacent  granite,  that  the  locality  must  be  regarded  as  worthy 
of  a  more  complete  examination  than  it  has  yet  received. 

WILLOWS  (G). 

The  Willows  occurrence  is  situated  east  of  Pretoria.  It 
consists  of  a  series  of  parallel  lodes — true  fissure-fillings — out- 
cropping in  clay-slates. 

The  ore  consists  of  chalcopyrite ,  pyrite,  and  tetrahedrite 
in  a  gangue  of  siderite.  In  the  oxidized  zone  where  the  ore 
contained  a  big  percentage  of  silver,  malachite  and  azurite 
largely  replaced  the  sulphides. 

A  great  deal  of  work  was  done  on  this  occurrence  in  the 
early  days  when  it  was  regarded  as  a  silver  mine,  and  it  is 
said  that  something  like  a  quarter  of  a  million's  worth  of  ore 
was  shipped  from  it. 

Development  work  has  lately  been  restarted  after  a  long 
period  of  idleness. 

VALLEY  (G). 

The  Valley  mine  is  located  85  miles  south-east  of  Bulawayo 
and  is  primarily  a  gold  proposition,  but  produces  copper  as  a 
bye-product. 


Copper.  -Jo 

It  is  on  a  quart/  nrf  situated  near  the  junction  of  the 
granite  and  schists,  and  is  heavily  charged  with  pyrite, 
nyrrhotite  and  chalcopyrite. 

In  the  middle  of  1906  a  smelter  was  erected  to  treat  the 
sulphide  concentrates,  the  resulting  matte  being  shipped.  The 
following  outputs  have  been  declared,  the  quantities  referring 
to  the  amount  of  copper  in  the  matte  : — 1906,  76  tons,  value 
£-5,765  ;  1907,  115  tons,  value  £9,625  ;  1908,  January  to  June 
inclusive,  40  tons,  value  £2,327. 

SUBENI  (G). 

Eunning  through  the  farms  Nonpareil,  Goudhoek,  and 
Fairview,  about  a  day's  journey  south-east  of  Vryheid,  is  a 
large  prominently  outcropping  quart/  reef,  situated  near,  and 
running  parallel  to,  the  junction  of  granite  and  schists. 

The  Subeni  prospect  is  located  on  Goudhoek  and  consists 
of  a  metalliferous  shoot  in  the  quart/  reef.  The  minerals  con- 
sist of  chalcopyrite  and  bornite  with  a  very  little  chalcocite. 

Shafts  have  also  been  put  down  in  this  reef  on  Nonpareil 
and  Fairview,  but  nothing  of  a  promising  nature  has  been  dis- 
covered at  those  places. 

OTAVI  (H). 

The  Otavi  Mountains  are  situated  in  northern  Damara- 
land.  The  writer  has  not  seen  the  copper  and  lead  deposits 
there,  the  following  account  being  extracted  from  a  paper*  by 
Kuntz.  The  mountains  are  composed  entirely  of  limestone. 

"  As  in  all  limestone  formations,  so  also  here  a  great 
number  of  caves  exist  as  a  consequence  of  the  action  of  water. 
Some  of  them  are  empty,  some  filled  with  water,  and  others 
filled  with  a  sandstone-like  mass,  which  contains  the  copper 
ore  in  small  and  large  pieces  of  different  shapes 

'  The     enormous     copper     deposits    at     Otavi,     Guchab, 
Tschumeb,  etc.,  are  simply  fillings  of  such  caves. 

*  J.  Kuntx,  Trans.  Geol.  Soc.  S.  Africa,  VII.,  p.  75-76  (1904). 


26  Ore  Deposits  of  South  Africa. 

"At  Gross-Ota vi,  dark  and  light  grey  limestone  can  be 
seen  cropping  out  in  layers,  striking  east  and  west,  and  dip- 
ping steeply  to  the  south.  This  rock  is  spangled  with  large 
and  small  pockets  of  copper  ore  of  irregular  shape,  and  from 
the  size  of  a  pea  to  that  of  a  cubic  yard.  The  pockets  again 
are  connected  with  each  other  by  a  network  of  irregular  run- 
ning veins,  as  the  accompanying  sketch  shows.  The  extent 
of  this  cavernous  zone  is  about  80  by  200  yards,  the  ore  con- 
sisting chiefly  of  chalcocite,  seldom  bornite  and  chalcopyrite. 
There  is  also  a  great  quantity  of  galenite. 

'  The  occurrences  at  Klein-Ota  vi  and  Guchab  are  of  the 
same  nature,  but  show  more  compact  bodies  of  ore. 

'  What  Gross-Otavi  is  on  a  small  scale,  Tschumeb  is 
on  a  much  larger  scale.  The  extent  of  the  outcrop  is  about 
200  yards  in  the  direction  of  the  east  and  west  strike,  and  40 
yards  vertically.  The  dip  is  steeply  to  the  south,  and  the  ore- 
deposit  seems  to  have  the  same  dip,  as  proved  by  the  prospect- 
ing shafts,  following,  apparently,  an  easier  decomposable  bed 
within  the  limestone. 

'  The  most  northerly  part  of  this  body  is .  to  a  great  extent , 
replaced  by  copper  and  lead  ore,  whilst  in  the  southern  part 
the  ore  occurs  breccia-like,  as  at  Gross-Otavi,  showing  that 
the  character  and  origin  of  the  deposits  are  the  same  in  both 
places. 

"  As  in  the  footwall,  so  in  the  hanging-wall  beds  of  lime- 
stone, numerous  small  clefts  and  cracks  are  filled  with  copper 
ore. 

"  On  the  eastern  part  of  the  outcrop  the  ore  consists  chiefly 
of  galenite,  the  copper  ore  being  in  the  minority.  Towards 
the  west  the  galenite  diminishes,  but  does  not  disappear 
entirely,  while  the  chalcocite  is  the  chief  ore  there.  Bornite 
and  chalcopyrite  exist  only  in  small  quantities." 

The  genesis  of  these  ore-bodies  seems  uncertain,  though 
it  is  clear  they  are  secondary  enrichments. 

During  the  financial  year  1907-8  (April  to  March    inclu- 


Copper.  27 

sivc)  there  were  shipped  from  Tschumeb  15,000  tons  of  ore 
with  an  average  content  of  0'035  per  cent,  silver,  19  per  cent, 
copper,  and  -23  per  cent.  lead.  Also  1,000  tons  of  matte  con- 
taining 0'040  per  cent,  of  silver,  38  per  cent,  of  copper,  and 
19  per  cent,  of  lead,  and  700  tons  of  95  per  cent,  lead  contain- 
ing 0'090  per  cent,  of  silver. 

From  Gnchab  1,800  tons  of  ore  containing  33  per  cent,  of 
copper  and  0'040  per  cent,  of  silver  were  shipped. 

During  the  first  quarter  of  the  current  financial  year  (April 
to  June,  1908)  about  6,500  tons  of  ore,  900  tons  of  matte  and 
800  tons  of  lead  were  shipped. 


CHAPTEK  V. 

COBALT. 
KRUISRIVIER   AND   EENZAMHE1D    (E  AND  D). 

THE  well-known  Kruisrivier  cobalt  lode  in  the  Selons 
Valley  has  often  been  described*  and  was  visited  by  the  writer 
in  1907.  The  mineral  is  smaltite,  which  occurs,  as  shown  in 
the  accompanying  section  (fig.  7),  as  an  impregnation  at  or 
near  the  junction  of  a  series  of  felspathic  quartzites  with  a 
gabbrodiorite  intrusion. 


FIG.  7. 
Section  through  Kruisrivier  Cobalt  Lode  (after  Mellor). 

In  the  best  exposure,  about  two  feet  from  and  parallel  to 
the  junction,  there  is  a  thin  interbedded  vein  of  smaltite  crys- 
tals on  either  side  of  which  the  quartzite  is  impregnated  with 

*  E.  J.  Dunn,  Quarterly  Journal  Geological  Society  of  London,  XXXIII.' 
p  883  .18771. 

K.  Oehmichen,  Zeitschrift  fur  Practische  Geologie,  p.  271-274  (1899). 

B.  Beck,  Trans  Geol   Soc.  S.  Africa,  X.,  p.  10  (1907). 

E.  T.  Mellor,  Eeport  Geol.  Survey,  Pretoria,  p.  70-71  (1907). 


Cobalt.  29 

the  same  mineral.  The  qnartzitc  in  this  zone  contains 
numerous  cherty  patches  and  has  a  generally  altered  and 
indurated  appearance. 

On  the  adjoining  farm,  Laatstedrift,  there  is  a  further 
occurrence  of  the  smaltite,  the  impregnated  rock  there  being 
the  gabbrodiorite. 

At  the  surface  the  smaltite  is  changed  into  the  peach-red 
and  black  oxidation  products,  erythrite  and  transvaalite.  Both 
contain  a  notable  quantity  of  gold,  up  to  four  ounces  per  ton. 

On  Eenzamheid,  about  a  mile  south  of  Balmoral  station, 
on  the  Pretoria-Delagoa  Bay  railway,  there  is  an  analogous 
occurrence  of  cobalt  ore,  which,  however,  the  writer  has  not 
seen.  It  has  been  described  by  several  observers.*  The 
smaltite,  together  with  a  little  quartz  and  molybdenite,  occurs 
as  an  impregnation  in  a  series  of  parallel  basic  dykes  that 
penetrate  a  highly  indurated  shale  and  are  probably  offshoots 
from  a  big  neighbouring  gabbrodiorite  intrusion.  A  small  pro- 
portion of  nickel  is  present  in  the  smaltite. 


*  D.  Dorftel,  Trans.  Geol.  Soc.  S  Africa,  VI   (1903). 
C.  B.  Horwood,  Trans.  Geol.  Soc.  S.  Africa,  VII.  (1904) 


CHAPTEE  VI. 

TIN. 
ROOI  BERG  (B,  C  AND  F). 

THE  Eooi  Berg,  which  give  their  name  to  this  field,  are  a 
group  of  hills  rising  out  of  the  flat  or  gently  undulating  country 
about  40  miles  due  west  of  the  village  of  Warmbaths.  The 
ore  occurrences  are  situated  mainly  on  the  northern  slopes  of 
the  Kooi  Berg  and  their  western  extension,  on  the  farms 
Haartebeestpoort,  Blauwbank,  Haartebeestfontein,  Olieven- 
bosch,  and  Onverwacht,  and  have  been  well  described  by 
Eecknagel.* 

The  whoje  of  the  area  is  covered  with  a  sedimentary  forma- 
tion consisting  of  felspathic  quartzites,  shales  and  thin  con- 
glomerates, surrounded  by,  and  resting  on,  granite,  which  is 
intrusive  into  it. 

Over  the  whole  north-western  half  of  this  area,  the  forma- 
tion is  practically  undisturbed,  and  strikes  from  north-west  to 
south-east,  writh  a  dip  of  5  to  10  degrees  to  the  north-east. 
Over  the  eastern  portion  of  this  area,  that  is,  on  the  northern 
slopes  of  the  Eooi  Berg,  the  formation  is  equally  free  from 
dislocations,  but  strikes  mainly  east  and  west,  with  a  southerly 
dip  of  about  30  degrees.  The  thickness  of  this  sedimentary 
formation  is  very  considerable,  amounting  to  several  thousand 
feet. 

Overlying  the  sedimentary  formation  are  a  series  of  felspar- 
porphyry  flows,  which  constitute  the  main  mass  of  the  Eooi 
Berg. 

*  K.  Kecknagel,  Trans   Geol.  Soc.  S.  Africa,  XI.  (1908). 


Tin.  31 

Throughout  the  whole  area  the  felspathic  quartzites  con- 
tain impregnations  of  tounnalino.  The  impregnations  vary 
from  very  small  isolated  irregular  patches  to  belts  which  follow 
the  same  stratum  uninterruptedly  for  long  distances.  With 
the  tourmaline  which  is  predominant  are  associated  the  follow- 
ing other  secondary  minerals,  in  decreasing  order  of  abund- 
ance, quartz,  felspar  (orthoclase) ,  and  sidero-calcite.  Cassi- 
terite  is  usually  present,  but  only  in  very  small  quantities. 
Pyrite  is  abundant,  but  chalcopyrite  is  rare. 

The  main  tin  occurrence  is  situated  on.  Haartebeestfontein 
and  Olievenbosch,  and  differs  somewhat  from  the  prevalent 
type.  It  may  be  described  as  a  gigantic  stockwork,  in  which 
the  tiny  cracks,  veinlets  and  impregnations  of  the  stockwork 
proper  are  represented  by  fissures  and  lodes  accompanied  by 
impregnations  of  corresponding  magnitude.  It  owes  its  redis- 
covery to  the  existence  over  the  whole  of  its  outcrop — some  70 
acres — of  very  extensive  old  workings. 

These  lodes  strike  in  all  directions,  but  at  least  two  parallel 
systems  can  be  distinguished  among  them,  one  containing 
lodes  with  an  approximately  north-south  strike,  and  the  other 
whose  lodes  strike  w^est-north-west. 

It  is  very  difficult  to  describe  in  a  few  words  the  various 
types  of  lodes  encountered,  as  there  is  scarcely  any  type  unre- 
presented. There  are  fissures  of  small  width,  mere  cracks 
with  no  vein  filling  except  some  clay ;  there  are  similar  fissures 
with  irregular  pockets  of  vein  minerals  attached  ;  there  are 
lodes  with  one  well-defined  wall,  showing  brecciated  structure 
and  enclosing  vughs  of  large  dimensions ;  and  there  are  lodes 
which  show  both  walls  well  defined  over  long  distances.  The 
task  of  conveying  a  proper  idea  of  the  various  types  is  made 
even  more  difficult  by  the  fact  that  in  most  of  the  lodes  various 
types  are  represented. 

The  filling  of  the  lodes  consists  in  part  of  decomposed 
country  rock  and  clay,  but  mainly  of  the  following  gangue 
minerals  (in  order  of  decreasing  abundance),  tourmaline, 


32  Ore  Deposits  of  South  Africa. 

quartz,  sidcro-calcite,  and  orthoclase.  As  an  accessory  gangiiL- 
mineral  found  in  small  quantity  only  fluorite  may  be  men- 
tioned. These  gangue  minerals  enclose,  besides  cassiterite  as 
the  main  metallic  mineral,  smaller  quantities  of  pyrite,  specu- 
larite,  chalcopyrite ,  galenite,  sphalerite,  and,  as  a  rarity,  gold. 
The  most  frequent  gangue  mineral,  and,  in  many  places,  the 
only  one,  is  tourmaline  ;  it  occurs  mostly  in  a  fibrous  state,  and 
very  often  forms  spherulitic  aggregates  which,  in  places,  are 
the  only  filling  of  wide  veins.  In  some  of  the  lodes  the  tour- 
maline crystals  form,  with  quartz  or  sidero-calcite,  a  dense 
felt-like  mixture  which,  only  under  the  microscope,  reveals  its 
true  nature.  Orthoclase,  mostly  pink  in  colour,  forms  the1 
gangue  mineral  in  several  lodes,  at  least  in  part,  and  usually 
appears  quite  fresh  to  the  unaided  eye,  but  in  places  can  be 
seen  changing  into  yellowish-green  sericite. 

The  cassiterite  occurs  either  by  itself,  forming  crystalline 
masses  of  smaller  extent  within  the  lodes,  or  associated  with 
either  of  the  gangue  minerals,  forming  coarse  to  fine-grained 
mixtures,  or  as  impregnations  in  the  country  rock. 

It  is  established  that  most  of  the  fissures  and  lodes  must 
have  been  originally  mere  cracks,  and  that  open  fissures  were 
the  exception.  The  present  apparent  filling  of  gangue 
minerals  is  merely  a  replacement  of  the  sides  of  these  cracks. 
It  is  further  established  that  a  rerupturing  of  many  fissures 
has  taken  place  afterwards,  as  the  brecciated  lodes  include 
fragments  of  pre-existing  "filling."  These  cracks  were  the 
main  channels  through  which  the  impregnating  agents  oper- 
ated. The  secondary  minerals  introduced  completely  replaced 
the  rock  immediately  adjoining  the  cracks,  but,  further  away. 
were  merely  disseminated  throughout  it,  and  finally  ceased  to 
penetrate  at  all.  It  is  interesting  to  note  that  where  these  im- 
pregnations followed  the  bedding  planes  a  banded  arrangement 
of  the  secondary  minerals  is  often  met  with. 

As  to  the  forces  that  produced  the  original  cracks,  no  satis- 
factory explanation  has  yet  been  offered.  It  is  certainly  most 


Tin.  33 

uncommon  to  find  an  area  of  sedimentary  formation,  practi- 
cally free  from  dislocation,  fissured  to  such  a  degree.  The 
only  dislocation  of  any  extent  is  that  which  separates  eastern 
and  western  are  as  above  described,  and  which  strikes  north- 
east through  Blauwbank,  but  this  is  three  miles  away  from  the 
fissured  area.  In  looking  for  possible  causes  of  the  fissuring, 
the  theory  of  torsional  movement  suggests  itself  as  a  feasible 
explanation. 

WEYNEK. 

The  Weynek  tin  lodes  are  situated  on  the  farm  of  that 
name  about  four  miles  from  the  great  Rooi  Berg  stockwork 
already  described.  They  are  in  the  same  felspathic  quartzites 
but  the  mode  of  occurrence  of  the  ore  is  very  different.  They, 
also,  owe  their  rediscovery  to  the  presence  of  old  workings  on 
them. 

There  are  two  main  parallel  lodes.  Their  mean  dip  is, 
approximately,  at  right  angles  to  that  of  the  felspathic 
quartzites,  but  their  course  here  and  there  follows  the  bedding 
planes  for  short  distances. 

The  filling  of  these  lodes  is  a  clayey  substance  of  uncer- 
tain nature.  The  cassiterite,  with  which  is  associated  the 
oxidation  products  of  chalcopyrite ,  is  disseminated  throughout 
this  filling.  No  tourmaline  is  present. 

The  genesis  of  the  ore  occurrence  is,  no  doubt,  the  same 
as  tha-t  of  the  Eooi  Berg,  and  the  lode  stuff  was  probably 
originally  very  similar,  but  has  subsequently  suffered  decom- 
position owing  to  local  causes. 

DOORNHOEK  (B  AND  F). 

The  farm  Doornhoek*  is  situated  north-east  of  Nylstroom. 
The  tin-ore  occurs  partly  as  an  irregular  impregnation  in  the 
granite,  but  chiefly  as  an  impregnation  along  fissures  in  the 
overlying  felspar-porphyry  flows. 

*  J.  P.  Johnson,  South  African  Mining  Journal,  VI.,  p.  571  (1908). 

D 


34  Ore  Deposits  of  South  Africa. 

The  main  ore-body,  which  has  been  traced  for  a  distance 
of  several  hundred  yards,  is  coincident  with  a  fissure  crossing 
the  felspar-porphyry  and  consists  of  an  impregnation  of  the 
crushed  rock  forming  the  walls.  Five  main  prospecting  shafts 
have  been  sunk  on  this  to  an  average  depth  of  about  fifty  feet, 
the  distance  between  the  two  extreme  end  shafts,  measuring 
along  the  lode,  being  1,250  feet.  The  average  width  of  this 
lode  is  about  eighteen  inches,  and  the  values  go  as  high  as  30 
per  cent,  tin,  and  average  10.  It  is  intersected  in  various 
directions  by  numerous  small  cassiterite-bearing  veins. 

This  lode  is  the  centre  of  a  highly  stanniferous  area  of  the 
nature  of  a  stock  work.  Numerous  parallel  veins  intersect  the 
surrounding  rock  in  all  directions. 

The  cassiterite  and  associated  secondary  minerals  (tour- 
maline, quartz,  sericite  and  fluorite),  while  megascopic  in  the 
granite,  are  in  a  very  fine  state  of  division  in  the  lodes,  being 
there  rarely  distinguishable  to  the  naked  eye. 

POTGIETERSRUST  (B). 

The  Potgietersrust  tin-field*  is  located  in  the  mountainous 
country  between  the  Magalakwin  River  and  its  tributary  the 
Sterk.  The  country  rock  is  granite.  Throughout  the  whole 
of  this  area  irregular  impregnations  of  cassiterite  and  associ- 
ated secondary  minerals  are  met  with,  but  the  payable  occur- 
rences so  far  dicovered  are  confined  to  a  zone  of  fissuring  run- 
ning through  the  farms  Solomonstempel,  Sterkwater,  Groen- 
fontein,  Eoodepoort,  and  Zaaiplaats. 

Running  north  and  south  through  Groenfontein  and  ex- 
tending into  Roodepoort  on  the  one  hand  and  Sterkwater  on 
the  other,  is  a  pegmatitic  quartz-vein,  dipping  at  a  low  angle 
to  the  west,  and  containing  massive  fluorite  and  sericite, 
together  with  much  coarsely  crystalline  cassiterite. 

The   granite    adjoining   this   quartz-vein   on   the    east   is 
*  J.  P.  Johnson,  Trans.  Geol.  Soc.  S.  Africa,  X.,  p.  115-119  (1907). 


Tin.  35 

sparsely  impregnated  with  small  crystals  of  cassiterite  for  a 
width  of  about  forty  yards.  Megascopically  this  impregnated 
granite  shows  little  evidence  of  alteration.  Apart  from  the 
cassiterite,  specks  of  yellow  to  green  sericite  and  an  occa- 
sional crystal  of  violet-blue  fluorite,  alone  differentiate  it  from 
the  adjoining  rock. 

East  of  this  belt  of  impregnated  granite  and  arranged  in 
a  line  roughly  parallel  to  it,  are  small  patches  of  similarly 
impregnated  granite  and  groups  of  peculiar  cylindrical  pipes. 
These  extend  into  Zaaiplaats  on  the  north  and  Solomons- 
tempel  on  the  south. 

The  cylindrical  pipes,  as  a  rule,  consist  of  a  core  of  altered 
granite  thickly  studded  with  crystals  of  cassiterite,  usually,  but 
not  always,  surrounded  by  a  sharply  defined  ring  consisting  of 
spherulites  (about  a  centimetre  in  diameter)  of  black  tourma- 
line closely  packed  in  a  white  quartz  matrix.  Outside  this  is 
a  halo  of  granite  characterized  by  specks  of  yellowish  green 
sericite  which  gradually  merges  into  the  unaltered  rock. 

The  extent  to  wrhich  the  granite  is  altered  in  these  pipes 
is  very  variable  in  one  and  the  same  pipe.  As  a  rule  the 
greater  part  of  the  rock  still  retains  its  granitic  texture,  and 
to  the  naked  eye  seems  to  have  suffered  little  alteration  beyond 
the  replacement  of  some  of  the  constituents  by  sericite, 
fluorite  and  cassiterite.  Much,  in  some  cases  the  major  por- 
tion, has,  on  the  other  hand,  suffered  such  complete  alteration 
as  to  be  megascopically  nothing  more  than  a  friable  dirty 
green  (sericitic)  rock  speckled  with  cassiterite.  Between 
these  two  extremes  every  gradation  can  be  observed.  Under 
the  microscope  the  sericite  is  seen  to  be  an  alteration  product 
of  the  felspars. 

Accessory  secondary  minerals  occurring  in  the  pipes  are 
molybdenite,  pyrite,  chalcopyrite  (and  hence  malachite), 
arsenopyrite,  and  galenite.  Grains  of  copper  were  also  found 
in  one  of  the  pipes  on  Zaaiplaats  embedded  in  the  tourmaline 
spherulites  at  a  depth  of  130  feet. 


36  Ore  Deposits  of  South  Africa. 

The  pipes  are  found  to  take  an  irregularly  inclined  course 
in  depth  and  sometimes  join  together.  They  mostly  dip  to 
the  west. 

The  payable  tin-ore  is  practically  confined  to  these  chim- 
neys. According  to  Merensky*  about  250  tons  of  ore  yielding 
an  average  of  30  per  cent,  of  tin  were  recovered  from  one  pipe 
alone. 

The  output  of  ore  and  concentrates  from  this  field  has  been 
as  follows  :—1906,  61  tons,  value  £3,928;  1907,  1,296  tons, 
value  £36,435;  1908,  January  to  June  inclusive,  712  tons, 
value  £43,754. 

WELBELOOND. 

This  is  an  interesting  prospect  situated  on  the  farm  Welbe- 
loond  on  the  eastern  flank  of  the  Tyger  Berg.  The  ore- 
occurrence,  which  I  have  not  seen,  and  for  a  description  of 
which  I  am  indebted  to  Mr.  Mills-Davies,  consists  of  small 
quartz  veins  carrying  coarse  cassiterite,  which  traverse  a  long 
low  kopje  of  slates  and  quartzites  not  far  from  an  intrusive 
granite  contact. 

EMBABAAN  (I). 

The  detrital  accumulations  of  cassiterite  in  the  Embabaan 
valley  have  been  regularly  worked  since  1892,  but  it  is  only 
from  1905  that  records  of  the  output  have  been  kept. 

The  cassiterite  is  derived  from  irregular  impregnations 
in  the  surrounding  granite.  Associated  with  the  cassiterite 
in  small  quantities  and  having  the  same  source  are  the  rare 
and  interesting  minerals,  aeschynite,  euxenite,  and  monazite. 

The  following  outputs  of  concentrates  have  been  regis- 
tered :— Year  ending  30th  June,  1906,  229  tons;  1907,  270 
tons;  1908,  512  tons. 


*  H.  Merensky,  Trans.  Geol.  Soc.  S.  Africa,  XL,  p.  35-39  (1908). 


Tin.  37 

LANGVERWACHT  (I). 

Detrital  accumulations  of  cassiterite  were  worked  in  the 
creek  on  the  farm  Langverwacht ,  about  twelve  miles  from 
Cape  Town,  during  1906  and  1907,  when  about  300  tons  of 
concentrates  were  shipped.  The  cassiterite  is  derived  from 
irregular  impregnations  in  the  surrounding  granite. 


CHAPTEE  VII. 

MOLYBDENUM. 
APPINGADAM  (B). 

THE  molybdenum  occurrence*  on  Appingadam  is  situated 
at  the  foot  of  the  north  end  of  the  kopjes  on  the  left  bank  of 
the  Sterk  River  and  occupies  an  irregularly  shaped  area. 
The  country  rock  around  this  area  is  granite  plentifully 
sprinkled  with  spherulites  of  tourmaline,  though,  so  far  as 
can  be  detected  by  the  unaided  eye,  otherwise  unaltered. 
Within  this  area  the  granite  shows  additional  alteration  of 
varying  intensity.  The  first  manifestations  are  the  increase 
in  the  number  of  tourmaline  spherulites  and  the  addition  of 
sericite  and  fluorite.  Then  molybdenite  and  cassiterite  (wrhich 
however  is  here  rare)  appear.  With  the  increase  in  quantity 
of  these  secondary  minerals  the  granite  gradually  loses  its 
original  character.  Finally,  in  certain  centres  of  intense 
alteration  peculiar  types  of  rock  have  been  produced  which 
bear  no  resemblance  to  the  original.  Their  constituent 
minerals  occur  both  in  evenly  granular  aggregates  and  as 
irregular  mixtures.  They  comprise  tourmaline-quartz  rock, 
sericite-quartz  rock,  sericite-molybdenite  rock,  molybdenite- 
arsenopyrite-quartz  rock,  and  arsenopyrite-fluorite  rock. 
Sphalerite  also  occurs  in  irregular  masses.  These  rock  types, 
of  course,  pass  one  into  the  other,  and  it  is  possible  to  get  hand 
specimens  containing  all  the  minerals  mentioned  together. 

Molybdenite  in  the  form  of  scales,  often  hexagonal,  is  the 
dominant  metallic  mineral.  The  yellow  oxidation  product, 
molybdite,  also  occurs. 

*  J.  P.  Johnson,  Trans.  Geol.  Soc.  S.  Africa,  X.,  p.  115-119  (1907). 


Molybdenum.  39 

An  interesting  feature  of  this  altered  area  is  the  presence 
of  groups  of  giant  spherulites  of  tourmaline,  some  as  much  as 
four  decimetres  in  diameter,  in  which  the  slender  needles  of 
the  ordinary  sized  spherulite  are  replaced  by  stout  columns. 
Tourmaline  also  occurs  as  short,  thick  crystals  associated  with 
quartz  in  small  irregular  bodies. 

To  find  a  market  molybdenum-ore  must  contain  not  less 
than  45  per  cent,  of  the  metal,  and  must  be  free  from  other 
metallic  minerals,  especially  copper.  Picked  sulphide,  practi- 
cally free  from  gangue,  will  command  from  16  to  19  shillings 
per  unit  per  cent,  of  molybdenum  per  ton. 

H  OUT  EN  BE  K  (B). 

On  the  farm  Houtenbek,  north-east  of  Pretoria,  there  are 
two  interesting  occurrences*  which,  although  not  of  economic 
importance,  are  deserving  of  mention  here.  The  country  rock 
is  granite. 

One  is  a  band,  two  feet  in  width,  consisting  of  almost  pure 
arsenopyrite,  some  fluorite,  and  large  leaves  of  molybdenite. 

The  other  is  a  similar  band  showing  arsenopyrite,  fluorite, 
and  large  crystals  of  monazite,  all  closely  intergrown.  The 
monazite  is  greenish-grey  to  dark  red  in  colour.  Some  of  the 
ore  was  reported  to  have  an  average  yield  of  5  per  cent,  of 
thorium. 


H.  Merensky,  Trans.  Geol.  Soc.  S.  Africa,  XI.,  p.  31  (1908). 


CHAPTEE  VIII. 

TUNGSTEN. 
RICHARDSON  (B). 

The  Richardson  mine  is  situated  at  Essexvale,  near  Bula- 
wayo. 

The  mineral  is  wolframite  with  a  little  scheelite,  and  occurs 
together  with  chlorite,  quartz,  tourmaline,  fluorite  and  topaz, 
as  an  impregnation  in  fine-grained  granite. 

In  1906  17  tons,  and  in  1907  12  tons,  of  picked  ore  were 
produced.  The  figures  of  output  for  the  first  six  months  of 
this  year  are  not  available. 

SCHEELITE-KING  (G). 

The  Scheelite-King  mine  is  situated  near  the  Umswege 
River,  on  the  Gaturna  side  of  the  Hartley  district. 

The  mineral  is  scheelite,  and  occurs  in  a  quartz  reef  in 
schistose  granite,  wThich  is  much  epidotized  in  the  vicinity. 

During  1907  38  tons  of  picked  ore  were  produced.  Mining 
is  being  continued  there,  but  no  declaration  of  output  has  yet 
been  made  for  this  year. 

I  am  indebted  to  Mr.  Mennell  for  the  information  relating 
to  the  above  occurrences  which  I  have  not  seen. 


CHAPTER  IX. 

LEAD. 
DWARSFONTEIN  (G). 

THE  Dwarsfontein  occurrence  is  situated  about  forty  miles 
south-east  of  Pretoria.  It  is  a  vertical  lode  outcropping  in 
clay-slates. 

The  ore  consists  of  galenite,  with  subordinate  chalcopyrite , 
pyrite,  and  tetrahedrite,  in  a  gangue  of  siderite.  In  the  oxi- 
dized zone  where  the  ore  contained  a  big  percentage  of  silver, 
cemssite  and  malachite  largely  replaced  the  sulphides. 

As  a  rarity,  translucent  red  crystals  of  the  lead  chromate, 
crocoite,  are  met  with. 

A  great  deal  of  work  was  done  on  this  occurrence  in  the 
early  days  when  it  was  regarded  as  a  silver  mine.  From  1891 
to  1893  inclusive,  £50,000  was  realized  from  the  sale  of  ore, 
concentrates,  and  metal. 

Work  has  lately  been  restarted.  A  small  amount  of  galenite 
was  contributed  to  the  Pretoria  district  output  (given  under 
Edendale)  for  the  first  six  months  of  this  year. 

ROODEKRANZ  (G). 

The  Roodekranz  occurrence  is  situated  sixteen  miles  south- 
west of  Pretoria.  It  is  a  lode  in  the  dolomite  formation  con- 
taining galenite  and  quartz. 

It  made  small  contributions  to  the  Pretoria  district  output 
during  1906  and  1907. 

PENHALONGA  (G). 

The  Penhalonga  mine  is  located  on  the  slopes  of  the  range 
of  hills  of  the  same  name,  some  10J  miles  north  of  Umtali.  It 


42  Ore  Deposits  of  South  Africa. 

is  primarily  a  gold  proposition,  but  produces  lead  as  a  bye- 
product.  A  short  description  of  it  has  been  published  by 
Townsend.*. 

The  ore  occurrence  is  a  vertical  quartz  reef  heavily  charged 
with  argentiferous  galenite,  running  in  and  parallel  to  a  series 
of  schists,  and  has  been  traced  for  a  distance  of  14,000  feet. 
There  are  numerous  old  workings  on  it. 

At  present  some  3,500  feet  of  the  lode  is  being  exploited. 
This  portion  is  divided  into  two  sections  by  a  longitudinal 
fault.  The  western  section  is  1,500  feet  in  length  and  the 
eastern  section  is  2,000  feet  in  length,  a  barren  stretch  of  700 
feet  separating  the  two.  The  lowest  drive  is  some  900  feet 
below  the  outcrop. 

The  ore-body  consists  of  quartz  lenticules  intercalated  with 
small  bands  of  schist.  Its  width  varies  from  25  to  50  feet,  of 
which  8  to  20  have  proved  to  be  worth  working.  The  quartz 
contains  in  addition  to  the  galenite  a  small  amount  of  zinc, 
iron  and  copper  sulphide.  The  oxidized  zone  is  characterized 
by  a  considerable  quantity  of  the  chromate  of  lead,  crocoite. 

In  the  eastern  section  there  is  a  parallel  band  of  quartz 
lenticules  lying  150  feet  to  the  south,  which,  however,  has  not 
been  found  in  the  western  section. 

Galenite  concentrates  have  been  shipped  for  several  years. 
From  January  to  June,  1908,  inclusive,  concentrates  contain- 
ing 510  tons  of  lead  valued  at  ,£6,292  were  produced. 

U1TLOOP-PJETFONTEIN   (F). 

This  occurrence!  is  situated  in  the  granite  ridge  which 
forms  the  boundary  between  the  farms  Uitloop  and  Eietfon- 
tein,  north  of  Potgietersrust ,  and  although  not  of  economic 
importance  is  of  special  interest  in  that  it  presents  the  unusual 

*  H.  P.  Townsend,  Journal  S.  African  Assoc.  Engineers,  XIIL,  p.  132-140 
(1908). 

f  J.  P.  Johnson,  Trans.  Geol.  Soc.  S.  Africa,  X.,  p.  115-119  (1907). 


Lead.  43 

feature  of  being-  genetically  similar  to  the  tin,  molybdenum 
and  tungsten  impregnations  already  described. 

The  main  occurrence  is  in  a  narrow  but  sharply  denned 
belt  of  altered  granite  which  strikes  approximately  north  and 
south  and  dips  at  a  big  angle  to  the  west.  This  is  traceable 
for  some  distance. 

The  section  is  as  follows  :  — 
(a)  Hanging  Wall. 

(6)  Mottled  Red  and  Yellowish-green  Rock.  Altered 
granite.  Granitic  structure  mostly  retained  but 
partly  obscure.  1  ft. 

(c)  Bluish-white   Chert-like   Rock.     Traversed  by  a 

network  of  fluorite  veinlets  containing  galenite. 
Altered  granite.  Traces  of  granitic  structure 
retained  in  places.  Specks  of  red  felspar 
throughout.  2  ft. 

(d)  Foot  Wall. 

A  short  distance  to  the  west  of  this  a  similar  occurrence 
runs  parallel  and  has  been  exposed  in  a  pit,  but  at  the  time  of 
my  visit  had  not  been  traced  far.  It  is  not  so  sharply  de- 
marcated from  the  enclosing  rock  as  in  the  main  occurrence. 
It  shows  veinlets  of  chert  and  galenite-bearing  fluorite  travers- 
ing a  narrow  belt  of  the  granite  which  is  not  megascopically 
much  altered. 

A  microscopical  examination  shows  the  ore-body  to  be  a 
line  of  crush  in  the  granite,  along  which  the  galenite  and  asso- 
ciated secondary  minerals  have  been  deposited  by  vapours  at 
the  expense  of  the  felspars. 

EDENDALE  (G). 

The  Edendale  lead  and  zinc  mines  are  located  on  the  farm 
of  the  same  name,  some  seventeen  miles  north-east  of  Pre- 
toria. Both  are  situated  on  a  lode  in  quartzites  and  shales. 

This  lode  consists  of  a  belt  of  fissuring  of  very  variable 
width.  It  has  been  traced  for  a  distance  of  about  2,000  yards 


44  Ore  Deposits  of  South  Africa. 

and  opened  up  at  one  point  to  a  depth  of  700  feet.  It  dips 
against  the  strata  at  an  angle  of  about  75  degrees. 

The  metalliferous  minerals  consist  of  galenite  and 
sphalerite  with  very  subordinate  chalcopyrite,  and  occurs  in 
the  form  of  lenticular  bodies  and  disseminations  throughout 
the  lode.  The  galenite  contains  about  14  ounces  of  silver  to 
the  ton,  and  constitutes  about  ten  per  cent,  of  the  ore.  The 
sphalerite  is  less  abundant.  The  two  minerals  are  seldom 
mixed. 

At  the  surface  the  sulphides  are  largely  replaced  by  the 
oxidation  products  cerussite  and  smithsonite.  In  smaller 
quantity  also  occur  the  red  oxide  of  lead,  minium,  in  opaque 
earthy  crusts,  and  translucent  yellowish-white  crystals  of  the 
lead  sulphate,  anglesite. 

These  mines  have  been  responsible  for  nearly  the  whole  of 
the  lead-ore  output  from  the  Pretoria  district,  which  is  as  fol- 
lows :—1904,  389  tons,  value  £'2,863;  1905,  768  tons,  value 
£3,756;  1906,  1,103  tons,  value  £14,473;  1907,  1,014  tons, 
value  £12,513;  1908,  January  to  June,  inclusive,  682  tons, 
value  £6,328.  During  1906  and  1907,  Eoodekranz  and, 
during  1908,  Dwarsfontein,  contributed  to  the  above  amounts. 

In  1907,  40  tons  of  zinc  ore,  valued  at  £365,  were  also 
sold. 

There  is  a  similar  but  smaller  occurrence  on  the  adjoining 
farms  Leuwkloof  and  Breeder stroom ,  some  28  miles  south- 
west of  Pretoria.  A  small  quantity  of  galenite  included  in  the 
above  figures  was  mined  from  there  in  1907  and  during  the 
first  six  months  of  1908. 

OTTOSHOOP  (H). 

The  Ottoshoop  lead  and  zinc  occurrences  are  analogous  to 
the  Otavi  copper-lead  deposits,  being  fillings  of  irregular  cavi- 
ties in  dolomite, 

The  ore  consists  of  galenite  and  sphalerite  in  a  gangue  of 
calcite,  fluorite  and  quartz.  A  little  chalcopyrite  is  sometimes 


Lead.  45 

associated  with  the  sphalerite.  The  immediately  adjoining 
rock  is  charged  with  tremolite. 

Pockets  have  been  located  on  the  farms,  Zendelingspost, 
Bufl'elshoek,  Witkop,  Naauwpoort,  Strydfontein,  Kuilfontein, 
Khenosterhoek,  and  Doornhoek. 

The  main  occurrence  at  Witkop  wrell  illustrates  these 
deposits.  There  a  horizontal  section  of  a  characteristic 
chamber  of  ore  is  exposed  at  the  surface.  It  is  approximately 
circular  in  outline,  and  has  a  diameter  of  about  fifty  yards. 
Within  this  area  the  solid  dolomite  is  replaced  by  angular 
blocks  embedded  in  a  matrix  of  calcite,  fluorite,  and  quartz 
containing  masses  amd  disseminations  of  galenite  and 
sphalerite.  The  galenite  is  argentiferous.  The  immediately 
surrounding  dolomite  and  the  included  blocks  of  same,  are 
charged  with  tremolite. 

At  the  surface  the  sulphides  are  often  changed  to  cerussite, 
smithsonite,  and  hemimorphite.  Minium  and  angiesite  are 
also  present  in  subordinate  quantity.  More  rarely  greenockite 
occurs  in  greenish  yellow  crusts. 

The  output  of  ore  from  this  district  has  been  : — 1907*, 
334  tons  of  lead  ore,  value  £3,930,  and  318  tons  of  zinc  ore, 
value  £2,851;  1908,  January  to  June  inclusive,  723  tons  of 
lead  ore,  value  £5,173,  and  249  tons  of  zinc  ore,  value  £1,715. 


*  Previous  records  unobtainable.  Some  ore  was  mined  at  Rhenosterhoek 
in  the  early  days. 


CHAPTEE   X. 

MEECUEY. 
LOUW'S  GREEK  (F). 

THIS  occurrence  is  about  ten  miles  south  of  Malelane  Sta- 
tion, on  the  Pretoria-Delagoa  Bay  Railway,  and  on  the  fur- 
ther side  of  the  range  of  hills  between  the  railway  and  Louw's 
Creek,  which  it  overlooks. 

The  writer  has  not  seen  the  ore-body  and  is  indebted  to 
Mr.  Draper  for  the  information  contained  in  this  note. 

The  mineral  is  cinnabar.  It  occurs  disseminated  through- 
out a  belt  of  crushed  quartzite  from  four  to  five  feet  wide  and 
about  400  yards  long.  The  mercury  content  over  this  width 
and  length  is  from  2  to  3  per  cent. 

Numerous  prospecting  shafts  have  been  put  down  on  it, 
but  an  adit  driven  lower  down  the  hill-side  to  intersect  the 
deposit  in  depth,  failed  to  locate  it. 


CHAPTEK  XI. 

ANTIMONY. 
MURCHISON  RANGE  (C). 

THE  Murchison  Eange*  is  made  up  of  picturesque  lines  of 
kopjes  lying  between  the  Selati  and  Groot-Letaba  Eivers.  It 
affords  a  pleasing  contrast  to  the  monotonously  flat  or  slightly 
undulating  bush-country  with  which  it  is  flanked.  Geologi- 
cally it  consists  of  a  narrow  belt  of  schists,  bounded  on  either 
side  by  granite. 

These  schists  in  common  with  their  representatives  in  other 
parts  of  the  country  contain  numerous  auriferous  quartz-veins, 
fahlbands  and  general  impregnations  of  a  lenticular  nature, 
which  conform  both  in  strike  and  dip  with  the  adjacent  beds. 
Sawyer  has  published  an  account  of  them  in  his  book  on  ' '  The 
Murchison  Eange"  (1892),  while  Wilson-Moore t ,  and,  more 
recently,  Merenskyt,  have  also  made  contributions  to  our 
knowledge  of  them. 

The  antimony  deposits  occur  in  the  string  of  kopjes  which 
extend  in  a  north-easterly  direction  from  north  of  Leydsdorp 
for  a  distance  of  about  thirty  miles.  These  kopjes  are  con- 
stituted of  a  definite  horizon  in  the  schists,  and  are  made  up 
of  much  altered  and  compressed  chloritic  quartzites  and  quartz 
and  clay  slates,  which  dip  at  a  big  angle  to  the  north.  The 
antimony  occurs  as  the  sulphide,  stibnite,  in  lenticular  veins 
which  can  be  traced  at  intervals  throughout  the  whole  length 
of  these  kopjes  and  which  seem  to  occupy  the  same  minor 
stratigraphical  position. 

*  J.  P.  Johnson,  The  Antimony  Deposits  of  the  Murchison  Range  (1907)" 
f  Wilson-Moore,  Trans.  Geol.  Soc.  S.  Africa,  I.,  p.  51-62  (1895). 
I  H.  Merenskv,  Trans.  Geol.  Soc.  S.  Africa,  VIII.,  p.  42-46  (1905). 


48  Ore  Deposits  of  South  Africa. 

The  richest  sections  of  the  antimony  reef  which  I  saw  are 
at  the  Gravelotte  and  Caledonian  properties,  but  very  little 
work  has  been  done  on  them,  and  that  only  in  the  oxidized 
zone.  There,  large  lenticules  of  pure  mineral  are  a  prominent 
feature,  while  gangue  is  absent  unless  an  occasional  small 
quartz  vein  can  be  considered  as  such. 

Only  at  the  Free-State  mine  has  the  antimony  reef  been 
opened  up  in  depth.  There  the  stibnite  occurs  disseminated 
throughout  a  calcareous  gangue,  which,  under  the  microscope, 
is  seen  to  be  mainly  made  up  of  calcite  and  dolomite  with  a 
little  green  sericite  and  some  secondary  quartz. 

It  seems  not  unlikely,  therefore,  that  these  antimony  de- 
posits are  impregnations  of  a  bed  of  metamorphosed  lime- 
stone.* 

At  the  Castle  kopjes  there  is  a  large  quartz  vein  which  was 
exceedingly  rich  in  visible  gold  in  places  and  w-hich  sometimes 
contains  a  little  stibnite. 

At  the  surface  the  stibnite  is  altered  into  a  yellow  oxide, 
probably  cervantite,  and  the  hydroxide,  stibiconite. 

The  antimony  reef  contains  a  notable  quantity  of  gold  and 
silver. 

The  individual  kopjes  of  the  antimony  line  are  separated 
from  one  another  by  transverse  faults,  the  beds  trending 
sharply  round  to  the  south  at  the  east  end  of  each  kopje,  and 
to  the  north  at  the  western  end. 


*  Mellor  in  a  later  account  (Report  Geol.  Survey,  Pretoria,  1907)  regards 
the  antimony  reef  as  a  true  lode. 


CHAPTEE  XII. 
IRON  (I). 

The  prominent  part  played  by  the  peculiar  banded  silica  - 
iron  oxide  rocks  and  magnetite-hematite-quartz  slates,  among 
the  older  sedimentary  formations  of  South  Africa  make  it 
probable  that  many  valuable  ore-bodies  will  be  found  where 
the  conditions  have  been  favourable  to  the  growth  of  secondary 
enrichments.  The  iron  wTas  probably  originally  deposited  in 
the  form  of  hydrate  at  the  time  of  the  laying  down  of  the  beds, 
and  though,  strictly  speaking,  largely  of  the  nature  of  a  chemi- 
cal precipitate,  may,  for  convenience,  be  regarded  as  a  mem- 
ber of  the  class  of  Detrital  Accumulations. 


FIG.  8. 

Section  through  the  Timeball  Rang^ntar  Pretoria,  showing  the  Magnetite 
Beds  (after  Hall  £  Steart). 

In  the  quartzite  of  the  Timeball  Kange,  near  Pretoria, 
there  are  two  beds  (fig.  8)  highly  charged  with  magnetite,  in 
one  of  which  the  magnetite  is  considerably  in  excess  of  the 
quartz  and  constitutes  a  good  iron  ore. 

The  iron  content  of  the  different  sedimentary  ores,  when 
theoretically  pure,  a  condition,  however,  which,  in  nature, 
is  never  attained  in  bulk,  are  : — Magnetite  (Fe3  04)  72  per 
cent.,  hematite,  and  its  variety  specularite  (Fe2  03),  70  per 
cent.,  limonite  (2Fe2  03.  3H2  0),  60  per  cent.,  and  siderite 
(Fe  C03)  48  per  cent.  The  most  serious  impurities  usually 
met  with  are  sulphur  and  phosphorus,  which  are  present  in  the 
form  of  pyrite  and  apatite,  and  must  not  exceed  certain 
minute  proportions. 

E 


CHAPTEE  XIII. 
HINTS  TO  PROSPECTORS. 

SOUTH  AFRICA  affords  a  fine  field  for  prospectors.  The 
number  of  competent  prospectors  in  the  country  could  prob- 
ably be  counted  on  the  fingers  of  one  hand,  a  circumstance, 
however,  which  one  can  only  expect.  A  combination  of 
mining  experience  and  geological  knowledge  are  the  essential 
qualifications  of  competent  prospectors,  and  those  possessing 
them  are  seldom  inclined  to  expend  their  talents  in  a  direc- 
tion where  reward  is  so  uncertain.  For  all  that  there  are 
prospectors. 

There  are  also  many  mining  men  possessing  the  necessary 
grounding,  who  now  and  then  can,  and  are  prepared  to,  spend 
a  little  time  and  money  in  trying  their  luck.  To  these 
amateur  prospectors  a  few  hints  will  be  useful. 

The  use  of  the  pan  in  detecting  the  presence  of  gold  or 
other  heavy  minerals  occurring  in  a  finely  divided  state  will 
already  be  familiar  to  them.  In  searching  for  coarse,  heavy 
minerals  in  loose  debris,  such  as  detrital  cassiterite,  a  sieve 
will  be  found  more  useful.  By  placing  some  of  the  debris 
inside  and  jigging  it  in  water,  the  heavier  minerals  will  be 
made  to  sink  to  the  bottom,  while  on  sharply  inverting  on 
the  ground  the  contents  will  be  spread  out  with  the  heavy 
minerals  on  top.  These  can  then  be  readily  examined. 

They  should  also  make  themselves  thoroughly  acquainted 
with  the  blowpipe  methods  of  determining  minerals  and  with 
the  use  of  the  more  readily  applicable  reagents.  These 
methods  are  described  in  all  text-books  on  minerals,  and  can- 
not be  detailed  here.  Proficiency  in  the  laboratory  is  essential 


Hints  to  Prospectors.  51 

before  their  aid  is  employed  in  the  field,  as  many  of  them  are 
not  so  easy  in  practice  as  they  seem  to  be  on  paper.  By  ex- 
perimenting with  mixtures  of  minerals  they  will  be  put  on 
their  guard  against  the  weaknesses  of  these  methods  under 
certain  conditions.  For  instance,  small  percentages  of  a 
mineral  yielding  a  very  strong  coloration  may  obscure  the 
presence  of  large  percentages  of  one  giving  only  a  feeble 
coloration. 

Amateur  prospectors  as  denned  by  the  writer  will  already 
be  familiar  with  the  commoner  rock-forming  minerals.  The 
economically  valuable  metalliferous  minerals,  with  the  excep- 
tion of  certain  iron  compounds,  however,  do  not  come  under 
that  heading.  The  writer  has,  therefore,  prepared  the  fol- 
lowing comparative  description  of  their  more  obvious  features. 
This  is  not  intended  to  supersede  the  use  of  a  text-book  on 
minerals,  but  merely  to  narrow  down  the  range  of  enquiry. 
It  must  be  remembered  that  numerous  other  minerals  will 
answer  to  many  of  the  descriptions  given  here,  though  they 
are  less  likely  to  occur  in  any  abundance,  and  it  must  be  re- 
membered that  the  most  precious  minerals  are  worthless  unless 
present  in  sufficient  quantity.  Having  located  the  mineral  in 
the  following  scheme  they  should,  unless  a  decisive  result 
such  as  the  production  of  a  recognisable  metallic  bead  shall 
have  been  obtained,  turn  to  their  text-book  and  put  it  through 
the  remaining  tests,  which  will  either  confirm  the  prelimin- 
ary identification,  or  else  prove  it  to  be  merely  a  similar  but 
worthless  mineral. 

ANTIMONY. 

Stibnite.— Sulphide  (Sb2  S3).  A  brittle  metallic  grey 
mineral.  Distinguishable  from  galenite  by  its  fusibility  in  an 
ordinary  flame. 

Antimony  is  used  in  the  manufacture  of  certain  bearing 
and  type  alloys.  Its  property  of  imparting  expansion  in 
changing  from  the  liquid  to  the  solid  state  and  hence  of  pro- 


52  Ore  Deposits  of  South  Africa. 

ducing  a  sharp  cast,  makes  it  specially  useful  for  the  latter 
purpose.     The  demand,  however,  is  limited. 

ARSENIC. 

Arsenopyrite  (Mispickel). — Sulpharsenide  of  Iron  (Fe 
As  S).  A  heavy  whitish  metallic  grey  mineral.  Resembles 
smaltite-chloantite.  It  is  the  main  source  of  arsenic  which, 
however,  is  alsov  obtained  as  a  bye-product  in  the  treatment  of 
the  cobalt-nickel  arsenides.  In  common  with  these  last 
minerals  it  readily  yields  the  characteristic  arsenical  (garlic- 
like)  odour  on  roasting  on  charcoal. 

CADMIUM. 

Greenockite.— Sulphide  (Cd  S).  A  yellow  translucent 
mineral.  It  is  a  common  associate  of  the  sulphide  of  zinc, 
sphalerite,  but  has  not  hitherto  been  known  to  occur  in  suffi- 
cient quantity  to  constitute  an  ore.  The  cadmium,  however, 
is  recoverable  as  a  bye-product  in  the  production  of  zinc. 

Cadmium  is  mainly  employed  in  the  manufacture  of  solder 
alloys.  Its  possession  of  the  quality  of  lowering  the  melting 
point  of  the  metals  with  which  it  combines,  makes  it  specially 
useful  for  that  purpose.  The  demand,  however,  is  limited. 

CHROMIUM. 

Chromite.— Oxide  of  chromium  and  iron  (Fe  Cr2  O4).  An 
opaque  black,  sometimes  brownish,  mineral.  Distinguishable 
from  magnetite  by  not  being  attracted  by  the  hand  magnet,  by 
its  brown  (as  opposed  to  black)  streak  and  by  the  test  for 
chromium.  Fuse  the  mineral  with  salt  of  phosphorus  when 
any  chromium  present  will  impart  a  dirty  green  colour  which 
changes  on  cooling  to  a  clear  glass. 

The  principal  uses  of  chromium  at  present  are  the  pre- 
paration of  chromium  salts,  employed  in  tanning  and  textile 
wrorks,  the  manufacture  of  chrome-brick  for  basic  furnace 
linings,  and  of  ferro-chrome  alloys  for  use  in  the  manufacture 
of  certain  steels. 


1<>  Prospector*.  53 

COBALT  AND  MCKEL. 

Smaltite-Chloantite. — Arsenides  of  cobalt  and  nickel 
[(Co  Ni)  As2]  respectively,  the  one  grading  into  the  other. 
A  heavy  whitish  metallic  grey  mineral.  Distinguishable  from 
arsenopyrite*  by  the  test  for  cobalt  or  nickel.  By  fusing  with 
borax,  after  first  roasting,  any  cobalt  present  will  impart  a 
deep  blue  colour  to  it.  Nickel  will  give,  when  using  the 
oxidizing  portion  of  the  flame,  a  violet  colour  which  changes 
to  reddish-brown  on  cooling. 

Cobaltite  and  Gcrsdorffite. — Sulpharsenides  of  cobalt 
(Co  As  S)  and  nickel  (Ni  As  S)  respectively,  are  exactly  simi- 
lar to  the  above  in  external  characters. 

Niccolitc. — Arsenide  (Ni  As).  A  heavy  reddish  metallic 
yellow  mineral,  much  resembling  pyrrhotite,  which,  however, 
will  attract  a  compass  needle,  and  bornite,  which  may  be  dis- 
tinguished by  the  test  for  copper.  It  usually  contains  a  little 
cobalt  and  sometimes  sulphur  or  antimony  in  place  of  some 
of  the  arsenic. 

Garnierite. — Hydrous  silicate  of  magnesium  and  nickel. 
An  earthy  apple-green  mineral  of  very  variable  composition. 
Not  known  in  South  Africa,  but  may  be  looked  for  in  serpen- 
tine areas.  Very  large  bodies  occur  in  that  rock  near  Noumea, 
New  Caledonia,  where  it  is  mixed  extensively. 

COPPER. 

Chalcocite. — Sulphide  (Cu2  S).  A  heavy,  brittle,  blackish 
metallic  grey  mineral.  Contains  80  per  cent,  copper. 

Bornite. — Sulphide  (Cu3  Fe  S3).  A  yellowish  to  reddish 
metallic  brown  mineral.  Contains  from  50  to  70  per  cent, 
copper.  Occupies  an  intermediate  position  between  chalco- 
cite  and  chalcopyrite,  and  is  usually  intimately  mixed  with 
varying  proportions  of  these  minerals,  hence  its  variable  cop- 

*  Arsenopyrite  itself,  however,  sometimes  contains  small  quantities  of 
hese  metals,  as  also  does  pyrrhotite,  pyrite  and  chalcopyrite  in  certain  occur- 
rences See  under  Nickel-ore  occurrences. 


54  Ore  Deposits  of  South  Africa. 

per  content  in  mass.  Distinguishable  from  both  these,  when 
isolated,  by  the  colour  on  a  fresh  fracture.  Readily  tarnishes 
to  metallic  red  and  blue  tints. 

Chalcopyrite.— Sulphide  (Cu  Fe  S2).  A  metallic  yellow 
mineral.  Contains  35  per  cent,  copper. 

Tetrahedrite. — Sulphantimonide  (Cu8  Sb2  S7).  A  brittle 
blackish  metallic  grey  mineral.  Often  argentiferous.  Has 
some  resemblance  to  chalcocite,  but  may  be  distinguished  by 
the  emission  of  dense  white  inodorous  antimonial  fumes  on 
roasting  on  charcoal.  The  tetrahedral  crystals  of  this  mineral 
afford  a  ready  basis  for  identification. 

Malachite  and  Azurite. — The  green  and  blue  carbonates, 
respectively,  resulting  from  the  alteration  of  the  sulphides. 
Sometimes  occur  in  translucent  crystals,  but  more  often  as  an 
opaque  earth.  They  form  important  ore-deposits. 

All  the  copper  minerals  will  yield  a  metallic  globule  under 
the  blowpipe  on  charcoal — the  sulphides  with  soda  after  roast- 
ing and  the  carbonates  without. 

LEAD. 

Galenite  (Galena).— Sulphide  (PbS).  A  heavy  brittle 
metallic  grey  mineral.  Eesembles  stibnite  and  argentite. 
Usually  contains  a  varying  quantity  of  silver. 

Cerussite. — Carbonate  (Pb  C  O3).  A  heavy  translucent 
white  mineral,  often  tinted  brown,  blue  or  green.  An  im- 
portant ore  of  lead  resulting  from  alteration  of  galenite. 

Both  minerals  yield  a  lead  globule  under  the  blowpipe  on 
charcoal. 

MANGANESE. 

Pyrolusite. — Oxide  (Mn  O2).  A  blackish  metallic  grey 
mineral. 

Psilomelane  and  Wad.— Hydrous  alteration  products  of 
pyrolusite  which  often  form  important  ore-deposits.  Eange 
from  a  hard  greyish-black  to  a  soft  brownish-black  mineral. 


Hints  to  Prospectors.  55 

These  minerals  may  be  distinguished  from  most  similar 
ones  by  the  test  for  manganese.  Fuse  with  soda  when  any 
manganese  present  will  impart  a  green  colour  to  it.  Wolfra- 
mite and  tantalite  will  yield  the  same  manganese  reaction,  but 

arc  much  heavier. 

MERCURY. 

Cinnabar. — Sulphide  (HgS).  A  very  heavy  translucent 
red  mineral.  Heated  in  a  glass  tube  it  gives  off  sulphurous 
fumes  and  mercury  which,  with  careful  manipulation,  can  be 
made  to  condense  on  the  sides. 

MOLYBDENUM. 

Molybdenite.— Sulphide  (MoS2).  A  flexible  metallic  grey 
mineral.  Distinguishable  from  graphite  by  dissolving  in  nitric 
acid  with  the  formation  of  a  wrhitish  grey  residue  (Mo03). 
Also  readily  gives  off  SO2  under  blowpipe. 

Molybdenum  is  mainly  employed  in  the  manufacture  of 
certain  steels,  to  which  it  imparts  desirable  properties. 

TANTALUM. 

Tantalite. — Tantalate  of  iron  and  manganese  [(Fe,  Mn) 
Ta2  04].  A  heavy  opaque  blackish-grey  mineral.  Eesembles 
cassiterite.  Distinguishable  from  wolframite  by  infusibility 
under  blowpipe  alone.  Not  known  to  occur  in  South  Africa, 
but  may  be  looked  for  with  tin-ore  occurrences. 

Tantalum  is  used  for  the  filaments  of  certain  electric  lamps. 
The  demand  is  very  limited. 

THORjfJM, 

Monazite.—  Phosphate  of  the  Rare  Metals  (Ce,  La,  Di, 
Y,  Er  and  Th).  Thorium  contents  very  variable.  A  trans- 
lucent yellowish-brown  to  hyacinth-red  mineral. 

Moistened  with  sulphuric  acid  and  ignited  on  a  platinum 


56  Ore  Deposits  of  South  Africa. 

wire,  it  affords  the  reaction  for  phosphoric  acid — a  bluish-green 
flame  coloration. 

Thorium  is  used  for  the  manufacture  of  gas  mantles.  The 
demand  is  very  limited. 

TIN. 

Cassiterite. — Oxide  (SnO).  A  heavy  opaque  black,  some- 
times translucent  brown,  mineral.  Resembles  some  varieties 
of  wolframite,  rutile,  sphalerite,  tourmaline  and  garnet.  Dis- 
tinguishable from  wolframite  by  infusibility  under  blowpipe 
alone.  From  the  others  by  its  high  specific  gravity.  From 
all  by  yielding  a  bead  of  tin  when  fused  with  soda  on  char- 
coal.* 

TUNGSTEN. 

Wolframite. — Tungstate  of  Iron  and  Manganese  [(Fe, 
Mn)  W  O4] .  A  heavy  opaque  greyish-black  or  brownish-black 
mineral.  Resembles  cassiterite  and  tantalite. 

Scheelite. — Tungstate  of  Calcium  (Ca  W  O4).  A  heavy 
translucent  white  and  yellow  mineral,  often  brownish, 
greenish  and  reddish. 

Tungsten  is  used  in  the  manufacture  of  certain  steels,  to 
which  it  imparts  desirable  properties. 

URANIUM. 

Uraninite  (Pitchblende). — A  very  heavy  opaque  greyish, 
greenish,  and  brownish-black  substance  of  variable  composi- 
tion, probably  including  two  or  three  different  mineral  species. 
Interesting  as  the  chief  source  from  wliich  radium  is  obtained. 

Uranium  has  been  used  experimentally  in  the  manufacture 
of  certain  steels  to  which  it  imparts  desirable  qualities,  but  it 
is  at  present  too  costly  for  that  purpose. 

*  This,  however,  is  not  readily  obtained  When  megascopic  by  wrapping 
in  a  piece  of  zinc  foil  and  dropping  into  hydrochloric  acid,  cassiterite  can  be 
reduced  externally  to  metal  which  will  reveal  its  characteristic  colour  on 
rubbing. 


57 

VANADIUM. 

Yanadinite. — A  chloro-vanadate  of  lead  (3Pb3  V2  08.  Pb 
Clo).  A  heavy  translucent  red  mineral.  Yields  a  globule  of 
lead  under  the  blowpipe  on  charcoal.  Distinguishable  from 
crocoite  and  minium  (which  are  also  a  different  tint  of  red) 
by  the  chlorine  test.  Heat  in  salt  of  phosphorus  bead  with 
copper  oxide,  when,  if  chlorine  is  present,  it  is  immediately 
surrounded  by  a  purple  flame. 

Vanadium  is  mostly  used  in  the  manufacture  of  certain 
steels  to  which  it  imparts  desirable  properties.  Vanadinite 
is  a  frequently  occurring  accessory  mineral  in  lead  ores  and 
was  formerly  concentrated  from  lead  ores  carrying  4  to  5  per 
cent,  in  the  Iberian  peninsula.  The  discovery  of  a  large 
occurrence  of  rich  vanadium  sulphide  (patronite)  in  South 
America,  has,  however,  for  the  time  being,  deprived  that 
source  of  commercial  value. 

ZINC. 

Sphalerite  (or  Zincblende).— Sulphide  (ZnS).  A  brittle 
translucent  yellowish  to  blackish-brown  mineral. 

Smitlisonite. — Carbonate  (ZnCo3).  A  brittle  translucent 
white,  often  greyish,  greenish  and  brownish,  sometimes  green, 
blue  and  brown,  mineral. 

Hemimorphite  (Calamine). — Hydrous  silicate  (H2  Zn  Si 
O5).  A  brittle  translucent  w^hite  mineral  sometimes  with  a 
bluish  or  greenish  tint,  also  yellow  to  brown. 

Smitlisonite  and  Hemimorphite  are  alteration  products  of 
sphalerite,  but  often  form  important  ore-deposits.  Under  the 
blowpipe  on  charcoal,  they  give  an  incrustation  which  is  yel- 
low while  hot  and  white  when  cold,  and  which,  when  heated 
again,  after  moistening  with  cobalt  solution,  becomes  a  fine 
green.  Smitlisonite  may  be  distinguished  from  the  others  by 
the  carbonate  reaction.* 

*  Effervescence  in  acids. 


58 


Ore  Deposits  of  South  Africa. 


Hints   to   Prospectors. 


59 


§•5 
^  S 


I -I 


60  Ore  Deposits  of  South  Africa. 

It  is  hardly  necessary  to  remind  the  reader  that  most 
minerals  exhibit  a  wide  range  of  •  variation  in  regard  to  their 
colour.  Only  that  most  frequently  met  with  is  mentioned  in 
the  above  comparative  descriptions.  Also  that  minerals  that 
are  usually  translucent  may  often  be  transparent  or  opaque. 

When  prospecting  for  the  insoluble  minerals  search 
should  always  first  be  made  among  the  detrital  accumulations 
in  the  bottom  of  creeks  or  in  the  natural  riffles  of  river  beds. 
On  finding  any  there,  they  may  be  traced  to  their  source  by 
panning  or  sieving  at  intervals  in  the  direction  from  which 
they  have  apparently  drifted.  In  prospecting  for  the  more 
readily  decomposable  minerals,  stains  of  the  colour  of  the 
oxidation  products  of  the  various  metalliferous  minerals  should 
be  looked  for,  especially  along  gossan-like  (rusty)  outcrops. 

Worthless  but  more  abundant  and  more  noticeable 
minerals  often  serve  as  indications  of  the  presence  of  valuable 
minerals.  Tourmaline,  for  instance,  nearly  always  points  to 
the  presence  of  cassiterite.  The  commoner  associates  of  the 
valuable  minerals  recorded  in  the  preceding  pages,  should  be 
specially  noted. 

Any  old  workings  that  may  be  discovered  should  be 
examined,  though  it  should  be  borne  in  mind  that  the  Kafirs 
very  often  excavated  small  patches  of  ore  that  would  not  pay 
a  European  to  exploit. 

Farther,  having  located  a  reef,  they  must  not  be  dis- 
couraged if,  after  tracing  it  for  a  little  distance,  they  find  its 
continuation  shifted  by  a  dyke  or  fault.  The  dislocation  pro- 
duced by  them  is  seldom  of  serious  extent.  A  little  per- 
severance will  usually  pick  up  the  reef  again  on  the  other  side. 

Finally,  it  may  be  observed  (1)  that  occurrences  of  tin  and 
its  genetically  allied  metals  should  be  looked  for  in  the  vicinity 
of  intrusive  granite  contacts,  (2)  that  gold  deposits  are  mostly 
found  among  the  schistose  rocks,  and  (3)  that  the  neighbour- 
hood of  big  gabbrodiorite  intrusions  is  frequently  prolific  in 
other  metalliferous  minerals. 


Ilintx    to    Pfospcctors,  61 

If  would-be  prospectors  will  ponder  over  this  last  state- 
ment for  a  little  while,  and  at  the  same  time  call  to  mind  the 
various  types  of  ore  occurrences  described  in  the  preceding 
pages,  and  the  working  hypothesis  by  which  they  are  co- 
ordinated, they  will  soon  realize  the  importance  to  them  of  au 
understanding  of  the  principles  outlined  in  the  introduction. 


BY  THE   SAME  AUTHOR. 

THE    STONE    IMPLEMENTS    OF 
SOUTH    AFRICA. 

SECOND  EDITION. 
REVISED  and  ENLARGED. 

WITH     ILLUSTRATIONS. 
Price  10  -. 


LONGMANS,     GREEN,     AND     COMPANY, 

39,   PATERNOSTER  ROW,   LONDON. 
NEW    YORK,    BOMBAY,    AND  CALCUTTA. 


THE  GEOLOGY  OF  THE  ROBERTS- 
VICTOR  DIAMOND  MINE. 

THE 

AURIFEROUS  CONGLOMERATES 
OF  THE  WITWATERSRAND. 

AND 

THE    ANTIMONY    DEPOSITS    OF 
THE   MURCHISON    RANGE. 

ILL  USTEATED, 
I/-  each. 


J.  S.  PHILLIPS,  121,  FLEET  STREET,  LONDON,  and 

J.    P.    JOHNSON,    JOHANNESBUKG. 


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